The patch titled Subject: mm: memcg: synchronize objcg lists with a dedicated spinlock has been added to the -mm tree. Its filename is mm-memcg-synchronize-objcg-lists-with-a-dedicated-spinlock.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-synchronize-objcg-lists-w... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-synchronize-objcg-lists-w...

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------------------------------------------------------ From: Roman Gushchin guro@fb.com Subject: mm: memcg: synchronize objcg lists with a dedicated spinlock

Alexander reported a circular lock dependency revealed by the mmap1 ltp test:

LOCKDEP_CIRCULAR (suite: ltp, case: mtest06 (mmap1)) WARNING: possible circular locking dependency detected 5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1 Not tainted ------------------------------------------------------ mmap1/202299 is trying to acquire lock: 00000001892c0188 (css_set_lock){..-.}-{2:2}, at: obj_cgroup_release+0x4a/0xe0 but task is already holding lock: 00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&sighand->siglock){-.-.}-{2:2}: __lock_acquire+0x604/0xbd8 lock_acquire.part.0+0xe2/0x238 lock_acquire+0xb0/0x200 _raw_spin_lock_irqsave+0x6a/0xd8 __lock_task_sighand+0x90/0x190 cgroup_freeze_task+0x2e/0x90 cgroup_migrate_execute+0x11c/0x608 cgroup_update_dfl_csses+0x246/0x270 cgroup_subtree_control_write+0x238/0x518 kernfs_fop_write_iter+0x13e/0x1e0 new_sync_write+0x100/0x190 vfs_write+0x22c/0x2d8 ksys_write+0x6c/0xf8 __do_syscall+0x1da/0x208 system_call+0x82/0xb0 -> #0 (css_set_lock){..-.}-{2:2}: check_prev_add+0xe0/0xed8 validate_chain+0x736/0xb20 __lock_acquire+0x604/0xbd8 lock_acquire.part.0+0xe2/0x238 lock_acquire+0xb0/0x200 _raw_spin_lock_irqsave+0x6a/0xd8 obj_cgroup_release+0x4a/0xe0 percpu_ref_put_many.constprop.0+0x150/0x168 drain_obj_stock+0x94/0xe8 refill_obj_stock+0x94/0x278 obj_cgroup_charge+0x164/0x1d8 kmem_cache_alloc+0xac/0x528 __sigqueue_alloc+0x150/0x308 __send_signal+0x260/0x550 send_signal+0x7e/0x348 force_sig_info_to_task+0x104/0x180 force_sig_fault+0x48/0x58 __do_pgm_check+0x120/0x1f0 pgm_check_handler+0x11e/0x180 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&sighand->siglock); lock(css_set_lock); lock(&sighand->siglock); lock(css_set_lock); *** DEADLOCK *** 2 locks held by mmap1/202299: #0: 00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180 #1: 00000001892ad560 (rcu_read_lock){....}-{1:2}, at: percpu_ref_put_many.constprop.0+0x0/0x168 stack backtrace: CPU: 15 PID: 202299 Comm: mmap1 Not tainted 5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1 Hardware name: IBM 3906 M04 704 (LPAR) Call Trace: [<00000001888aacfe>] dump_stack_lvl+0x76/0x98 [<0000000187c6d7be>] check_noncircular+0x136/0x158 [<0000000187c6e888>] check_prev_add+0xe0/0xed8 [<0000000187c6fdb6>] validate_chain+0x736/0xb20 [<0000000187c71e54>] __lock_acquire+0x604/0xbd8 [<0000000187c7301a>] lock_acquire.part.0+0xe2/0x238 [<0000000187c73220>] lock_acquire+0xb0/0x200 [<00000001888bf9aa>] _raw_spin_lock_irqsave+0x6a/0xd8 [<0000000187ef6862>] obj_cgroup_release+0x4a/0xe0 [<0000000187ef6498>] percpu_ref_put_many.constprop.0+0x150/0x168 [<0000000187ef9674>] drain_obj_stock+0x94/0xe8 [<0000000187efa464>] refill_obj_stock+0x94/0x278 [<0000000187eff55c>] obj_cgroup_charge+0x164/0x1d8 [<0000000187ed8aa4>] kmem_cache_alloc+0xac/0x528 [<0000000187bf2eb8>] __sigqueue_alloc+0x150/0x308 [<0000000187bf4210>] __send_signal+0x260/0x550 [<0000000187bf5f06>] send_signal+0x7e/0x348 [<0000000187bf7274>] force_sig_info_to_task+0x104/0x180 [<0000000187bf7758>] force_sig_fault+0x48/0x58 [<00000001888ae160>] __do_pgm_check+0x120/0x1f0 [<00000001888c0cde>] pgm_check_handler+0x11e/0x180 INFO: lockdep is turned off.

In this example a slab allocation from __send_signal() caused a refilling and draining of a percpu objcg stock, resulted in a releasing of another non-related objcg. Objcg release path requires taking the css_set_lock, which is used to synchronize objcg lists.

This can create a circular dependency with the sighandler lock, which is taken with the locked css_set_lock by the freezer code (to freeze a task).

In general it seems that using css_set_lock to synchronize objcg lists makes any slab allocations and deallocation with the locked css_set_lock and any intervened locks risky.

To fix the problem and make the code more robust let's stop using css_set_lock to synchronize objcg lists and use a new dedicated spinlock instead.

Link: https://lkml.kernel.org/r/Yfm1IHmoGdyUR81T@carbon.dhcp.thefacebook.com Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") Signed-off-by: Roman Gushchin guro@fb.com Reported-by: Alexander Egorenkov egorenar@linux.ibm.com Tested-by: Alexander Egorenkov egorenar@linux.ibm.com Reviewed-by: Waiman Long longman@redhat.com Acked-by: Tejun Heo tj@kernel.org Reviewed-by: Shakeel Butt shakeelb@google.com Reviewed-by: Jeremy Linton jeremy.linton@arm.com Tested-by: Jeremy Linton jeremy.linton@arm.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/memcontrol.h | 5 +++-- mm/memcontrol.c | 10 +++++----- 2 files changed, 8 insertions(+), 7 deletions(-)

--- a/include/linux/memcontrol.h~mm-memcg-synchronize-objcg-lists-with-a-dedicated-spinlock +++ a/include/linux/memcontrol.h @@ -219,7 +219,7 @@ struct obj_cgroup { struct mem_cgroup *memcg; atomic_t nr_charged_bytes; union { - struct list_head list; + struct list_head list; /* protected by objcg_lock */ struct rcu_head rcu; }; }; @@ -315,7 +315,8 @@ struct mem_cgroup { #ifdef CONFIG_MEMCG_KMEM int kmemcg_id; struct obj_cgroup __rcu *objcg; - struct list_head objcg_list; /* list of inherited objcgs */ + /* list of inherited objcgs, protected by objcg_lock */ + struct list_head objcg_list; #endif

MEMCG_PADDING(_pad2_); --- a/mm/memcontrol.c~mm-memcg-synchronize-objcg-lists-with-a-dedicated-spinlock +++ a/mm/memcontrol.c @@ -254,7 +254,7 @@ struct mem_cgroup *vmpressure_to_memcg(s }

#ifdef CONFIG_MEMCG_KMEM -extern spinlock_t css_set_lock; +static DEFINE_SPINLOCK(objcg_lock);

bool mem_cgroup_kmem_disabled(void) { @@ -298,9 +298,9 @@ static void obj_cgroup_release(struct pe if (nr_pages) obj_cgroup_uncharge_pages(objcg, nr_pages);

- spin_lock_irqsave(&css_set_lock, flags); + spin_lock_irqsave(&objcg_lock, flags); list_del(&objcg->list); - spin_unlock_irqrestore(&css_set_lock, flags); + spin_unlock_irqrestore(&objcg_lock, flags);

percpu_ref_exit(ref); kfree_rcu(objcg, rcu); @@ -332,7 +332,7 @@ static void memcg_reparent_objcgs(struct

objcg = rcu_replace_pointer(memcg->objcg, NULL, true);

- spin_lock_irq(&css_set_lock); + spin_lock_irq(&objcg_lock);

/* 1) Ready to reparent active objcg. */ list_add(&objcg->list, &memcg->objcg_list); @@ -342,7 +342,7 @@ static void memcg_reparent_objcgs(struct /* 3) Move already reparented objcgs to the parent's list */ list_splice(&memcg->objcg_list, &parent->objcg_list);

- spin_unlock_irq(&css_set_lock); + spin_unlock_irq(&objcg_lock);

percpu_ref_kill(&objcg->refcnt); } _

Patches currently in -mm which might be from guro@fb.com are

mm-memcg-synchronize-objcg-lists-with-a-dedicated-spinlock.patch