The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80
but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0
-> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe
other info that might help us debug this:
Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem
Possible unsafe locking scenario:
CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241);
*** DEADLOCK ***
No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com --- mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
- mem_hotplug_begin(); - /* * All memory blocks must be offlined before removing memory. Check * whether all memory blocks in question are offline and return error @@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
- /* remove memory block devices before removing memory */ + /* + * Remove memory block devices before removing memory, and do + * not hold the mem_hotplug_lock() over kobject removal + * operations. lock_device_hotplug() keeps the + * check_memblock_offlined_cb result valid until the entire + * removal process is complete. + */ remove_memory_block_devices(start, size);
+ mem_hotplug_begin(); + arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);
On 10.01.20 05:30, Dan Williams wrote:
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK ***No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) BUG_ON(check_hotplug_memory_range(start, size));
- mem_hotplug_begin();
- /*
- All memory blocks must be offlined before removing memory. Check
- whether all memory blocks in question are offline and return error
@@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
- /* remove memory block devices before removing memory */
- /*
* Remove memory block devices before removing memory, and do* not hold the mem_hotplug_lock() over kobject removal* operations. lock_device_hotplug() keeps the* check_memblock_offlined_cb result valid until the entire* removal process is complete.*/
Maybe shorten that to
/* * Remove memory block devices before removing memory. Protected * by the device_hotplug_lock only. */
AFAIK, the device hotplug lock is sufficient here. The memory hotplug lock / cpu hotplug lock is only needed when calling into arch code (especially for PPC). We hold both locks when onlining/offlining memory.
remove_memory_block_devices(start, size);
- mem_hotplug_begin();
- arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);
I'd suggest to do the same in the adding part right away (if easily possible) to make it clearer. I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 05:30, Dan Williams wrote:
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK ***No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
mem_hotplug_begin();/* * All memory blocks must be offlined before removing memory. Check * whether all memory blocks in question are offline and return error@@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
/* remove memory block devices before removing memory */
/** Remove memory block devices before removing memory, and do* not hold the mem_hotplug_lock() over kobject removal* operations. lock_device_hotplug() keeps the* check_memblock_offlined_cb result valid until the entire* removal process is complete.*/Maybe shorten that to
/*
- Remove memory block devices before removing memory. Protected
- by the device_hotplug_lock only.
*/
Why make someone dig for the reasons this lock is sufficient?
AFAIK, the device hotplug lock is sufficient here. The memory hotplug lock / cpu hotplug lock is only needed when calling into arch code (especially for PPC). We hold both locks when onlining/offlining memory.
remove_memory_block_devices(start, size);
mem_hotplug_begin();arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);I'd suggest to do the same in the adding part right away (if easily possible) to make it clearer.
Let's let this fix percolate upstream for a bit to make sure there was no protection the mem_hotplug_begin() was inadvertently providing.
I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
I see that, but I prefer lockdep_assert_held() in the code rather than comments. I'll send a patch to fix that up.
On 10.01.20 17:42, Dan Williams wrote:
On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 05:30, Dan Williams wrote:
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK ***No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
mem_hotplug_begin();/* * All memory blocks must be offlined before removing memory. Check * whether all memory blocks in question are offline and return error@@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
/* remove memory block devices before removing memory */
/** Remove memory block devices before removing memory, and do* not hold the mem_hotplug_lock() over kobject removal* operations. lock_device_hotplug() keeps the* check_memblock_offlined_cb result valid until the entire* removal process is complete.*/Maybe shorten that to
/*
- Remove memory block devices before removing memory. Protected
- by the device_hotplug_lock only.
*/
Why make someone dig for the reasons this lock is sufficient?
I think 5 LOC of comment are too much for something that is documented e.g., in Documentation/core-api/memory-hotplug.rst ("Locking Internals"). But whatever you prefer.
AFAIK, the device hotplug lock is sufficient here. The memory hotplug lock / cpu hotplug lock is only needed when calling into arch code (especially for PPC). We hold both locks when onlining/offlining memory.
remove_memory_block_devices(start, size);
mem_hotplug_begin();arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);I'd suggest to do the same in the adding part right away (if easily possible) to make it clearer.
Let's let this fix percolate upstream for a bit to make sure there was no protection the mem_hotplug_begin() was inadvertently providing.
Yeah, why not.
I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
I see that, but I prefer lockdep_assert_held() in the code rather than comments. I'll send a patch to fix that up.
That won't work as early boot code from ACPI won't hold it while it adds memory. And we decided (especially Michal :) ) to keep it like that.
On 10.01.20 17:54, David Hildenbrand wrote:
On 10.01.20 17:42, Dan Williams wrote:
On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 05:30, Dan Williams wrote:
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK ***No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
mem_hotplug_begin();/* * All memory blocks must be offlined before removing memory. Check * whether all memory blocks in question are offline and return error@@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
/* remove memory block devices before removing memory */
/** Remove memory block devices before removing memory, and do* not hold the mem_hotplug_lock() over kobject removal* operations. lock_device_hotplug() keeps the* check_memblock_offlined_cb result valid until the entire* removal process is complete.*/Maybe shorten that to
/*
- Remove memory block devices before removing memory. Protected
- by the device_hotplug_lock only.
*/
Why make someone dig for the reasons this lock is sufficient?
I think 5 LOC of comment are too much for something that is documented e.g., in Documentation/core-api/memory-hotplug.rst ("Locking Internals"). But whatever you prefer.
AFAIK, the device hotplug lock is sufficient here. The memory hotplug lock / cpu hotplug lock is only needed when calling into arch code (especially for PPC). We hold both locks when onlining/offlining memory.
remove_memory_block_devices(start, size);
mem_hotplug_begin();arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);I'd suggest to do the same in the adding part right away (if easily possible) to make it clearer.
Let's let this fix percolate upstream for a bit to make sure there was no protection the mem_hotplug_begin() was inadvertently providing.
Yeah, why not.
I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
I see that, but I prefer lockdep_assert_held() in the code rather than comments. I'll send a patch to fix that up.
That won't work as early boot code from ACPI won't hold it while it adds memory. And we decided (especially Michal :) ) to keep it like that.
Was only thinking about the adding part, it could work for the removal part, though :)
On Fri, Jan 10, 2020 at 8:54 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 17:42, Dan Williams wrote:
On Fri, Jan 10, 2020 at 1:10 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 05:30, Dan Williams wrote:
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock().
The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices().
The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, so it is left alone for now.
This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()).
====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK ***No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations.
Cc: stable@vger.kernel.org Cc: Vishal Verma vishal.l.verma@intel.com Cc: David Hildenbrand david@redhat.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Michal Hocko mhocko@suse.com Cc: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
mm/memory_hotplug.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 55ac23ef11c1..a4e7dadded08 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
mem_hotplug_begin();/* * All memory blocks must be offlined before removing memory. Check * whether all memory blocks in question are offline and return error@@ -1777,9 +1775,17 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size) /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");
/* remove memory block devices before removing memory */
/** Remove memory block devices before removing memory, and do* not hold the mem_hotplug_lock() over kobject removal* operations. lock_device_hotplug() keeps the* check_memblock_offlined_cb result valid until the entire* removal process is complete.*/Maybe shorten that to
/*
- Remove memory block devices before removing memory. Protected
- by the device_hotplug_lock only.
*/
Why make someone dig for the reasons this lock is sufficient?
I think 5 LOC of comment are too much for something that is documented e.g., in Documentation/core-api/memory-hotplug.rst ("Locking Internals"). But whatever you prefer.
Sure, lets beef up that doc to clarify this case and refer to it.
AFAIK, the device hotplug lock is sufficient here. The memory hotplug lock / cpu hotplug lock is only needed when calling into arch code (especially for PPC). We hold both locks when onlining/offlining memory.
remove_memory_block_devices(start, size);
mem_hotplug_begin();arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size);I'd suggest to do the same in the adding part right away (if easily possible) to make it clearer.
Let's let this fix percolate upstream for a bit to make sure there was no protection the mem_hotplug_begin() was inadvertently providing.
Yeah, why not.
I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
I see that, but I prefer lockdep_assert_held() in the code rather than comments. I'll send a patch to fix that up.
That won't work as early boot code from ACPI won't hold it while it adds memory. And we decided (especially Michal :) ) to keep it like that.
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
Why make someone dig for the reasons this lock is sufficient?
I think 5 LOC of comment are too much for something that is documented e.g., in Documentation/core-api/memory-hotplug.rst ("Locking Internals"). But whatever you prefer.
Sure, lets beef up that doc to clarify this case and refer to it.
Referring is a good idea. We should change the "is advised" for the device_online() to a "is required" or similar. Back then I wasn't sure how it all worked in detail...
I properly documented the semantics of add_memory_block_devices()/remove_memory_block_devices() already (that they need the device hotplug lock).
I see that, but I prefer lockdep_assert_held() in the code rather than comments. I'll send a patch to fix that up.
That won't work as early boot code from ACPI won't hold it while it adds memory. And we decided (especially Michal :) ) to keep it like that.
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand david@redhat.com wrote: [..]
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
That's a large comment block when we could have just taken the lock. There's probably many other code paths in the kernel where some locks are not necessary before userspace is up, but the code takes the lock anyway to minimize the code maintenance burden. Is there really a compelling reason to be clever here?
On 10.01.20 18:33, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand david@redhat.com wrote: [..]
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
That's a large comment block when we could have just taken the lock. There's probably many other code paths in the kernel where some locks are not necessary before userspace is up, but the code takes the lock anyway to minimize the code maintenance burden. Is there really a compelling reason to be clever here?
It was a lengthy discussion back then and I was sharing your opinion. I even had a patch ready to enforce that we are holding the lock (that's how I identified that specific case in the first place).
On Fri, Jan 10, 2020 at 9:36 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 18:33, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand david@redhat.com wrote: [..]
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
That's a large comment block when we could have just taken the lock. There's probably many other code paths in the kernel where some locks are not necessary before userspace is up, but the code takes the lock anyway to minimize the code maintenance burden. Is there really a compelling reason to be clever here?
It was a lengthy discussion back then and I was sharing your opinion. I even had a patch ready to enforce that we are holding the lock (that's how I identified that specific case in the first place).
Ok, apologies I missed that opportunity to back you up. Michal, is this still worth it?
On 10.01.20 18:39, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:36 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 18:33, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand david@redhat.com wrote: [..]
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
That's a large comment block when we could have just taken the lock. There's probably many other code paths in the kernel where some locks are not necessary before userspace is up, but the code takes the lock anyway to minimize the code maintenance burden. Is there really a compelling reason to be clever here?
It was a lengthy discussion back then and I was sharing your opinion. I even had a patch ready to enforce that we are holding the lock (that's how I identified that specific case in the first place).
Ok, apologies I missed that opportunity to back you up. Michal, is this still worth it?
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
On Fri, Jan 10, 2020 at 9:42 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 18:39, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:36 AM David Hildenbrand david@redhat.com wrote:
On 10.01.20 18:33, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand david@redhat.com wrote: [..]
So then the comment is actively misleading for that case. I would expect an explicit _unlocked path for that case with a comment about why it's special. Is there already a comment to that effect somewhere?
__add_memory() - the locked variant - is called from the same ACPI location either locked or unlocked. I added a comment back then after a longe discussion with Michal:
drivers/acpi/scan.c: /* * Although we call __add_memory() that is documented to require the * device_hotplug_lock, it is not necessary here because this is an * early code when userspace or any other code path cannot trigger * hotplug/hotunplug operations. */
It really is a special case, though.
That's a large comment block when we could have just taken the lock. There's probably many other code paths in the kernel where some locks are not necessary before userspace is up, but the code takes the lock anyway to minimize the code maintenance burden. Is there really a compelling reason to be clever here?
It was a lengthy discussion back then and I was sharing your opinion. I even had a patch ready to enforce that we are holding the lock (that's how I identified that specific case in the first place).
Ok, apologies I missed that opportunity to back you up. Michal, is this still worth it?
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
Oh, now I see the problem. You need to add that lock so far away from the __add_memory() to avoid lock inversion problems with the acpi_scan_lock. The organization I was envisioning would not work without deeper refactoring.
On Fri 10-01-20 13:27:24, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:42 AM David Hildenbrand david@redhat.com wrote:
[...]
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
Oh, now I see the problem. You need to add that lock so far away from the __add_memory() to avoid lock inversion problems with the acpi_scan_lock. The organization I was envisioning would not work without deeper refactoring.
Sorry to come back to this late. Has this been resolved?
On Fri, Jan 24, 2020 at 4:56 AM Michal Hocko mhocko@kernel.org wrote:
On Fri 10-01-20 13:27:24, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:42 AM David Hildenbrand david@redhat.com wrote:
[...]
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
Oh, now I see the problem. You need to add that lock so far away from the __add_memory() to avoid lock inversion problems with the acpi_scan_lock. The organization I was envisioning would not work without deeper refactoring.
Sorry to come back to this late. Has this been resolved?
The mem_hotplug_lock lockdep splat fix in this patch has not landed. David and I have not quite come to consensus on how to resolve online racing removal. IIUC David wants that invalidation to be pages_correctly_probed(), I would prefer it to be directly tied to the object, struct memory_block, that remove_memory_block_devices() has modified, mem->section_count = 0.
...or are you referring to the discussion about acpi_scan_lock()? I came around to agreeing with your position that documenting was better than adding superfluous locking especially because the acpi_scan_lock() is take so far away from where the device_hotplug lock is needed.
On 24.01.20 19:04, Dan Williams wrote:
On Fri, Jan 24, 2020 at 4:56 AM Michal Hocko mhocko@kernel.org wrote:
On Fri 10-01-20 13:27:24, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:42 AM David Hildenbrand david@redhat.com wrote:
[...]
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
Oh, now I see the problem. You need to add that lock so far away from the __add_memory() to avoid lock inversion problems with the acpi_scan_lock. The organization I was envisioning would not work without deeper refactoring.
Sorry to come back to this late. Has this been resolved?
The mem_hotplug_lock lockdep splat fix in this patch has not landed. David and I have not quite come to consensus on how to resolve online racing removal. IIUC David wants that invalidation to be pages_correctly_probed(), I would prefer it to be directly tied to the object, struct memory_block, that remove_memory_block_devices() has modified, mem->section_count = 0.
FWIW, there is no such race possible - esp. zombie devices (see https://lore.kernel.org/lkml/1580c2bb-5e94-121d-8153-c8a7230b764b@redhat.com...).
(I'm planning to send a patch to remove mem->section_count soon)
On Fri 24-01-20 10:04:52, Dan Williams wrote:
On Fri, Jan 24, 2020 at 4:56 AM Michal Hocko mhocko@kernel.org wrote:
On Fri 10-01-20 13:27:24, Dan Williams wrote:
On Fri, Jan 10, 2020 at 9:42 AM David Hildenbrand david@redhat.com wrote:
[...]
For your reference (roughly 5 months ago, so not that old)
https://lkml.kernel.org/r/20190724143017.12841-1-david@redhat.com
Oh, now I see the problem. You need to add that lock so far away from the __add_memory() to avoid lock inversion problems with the acpi_scan_lock. The organization I was envisioning would not work without deeper refactoring.
Sorry to come back to this late. Has this been resolved?
The mem_hotplug_lock lockdep splat fix in this patch has not landed. David and I have not quite come to consensus on how to resolve online racing removal. IIUC David wants that invalidation to be pages_correctly_probed(), I would prefer it to be directly tied to the object, struct memory_block, that remove_memory_block_devices() has modified, mem->section_count = 0.
I was asking about this part and I can see you have already posted a patch[1] and I do not see any reason for not merging it.
[1] http://lkml.kernel.org/r/157991441887.2763922.4770790047389427325.stgit@dwil...
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