From: John Keeping john@metanate.com
[ Upstream commit 401e4963bf45c800e3e9ea0d3a0289d738005fd4 ]
With CONFIG_PREEMPT_RT, it is possible to hit a deadlock between two normal priority tasks (SCHED_OTHER, nice level zero):
INFO: task kworker/u8:0:8 blocked for more than 491 seconds. Not tainted 5.15.49-rt46 #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/u8:0 state:D stack: 0 pid: 8 ppid: 2 flags:0x00000000 Workqueue: writeback wb_workfn (flush-7:0) [<c08a3a10>] (__schedule) from [<c08a3d84>] (schedule+0xdc/0x134) [<c08a3d84>] (schedule) from [<c08a65a0>] (rt_mutex_slowlock_block.constprop.0+0xb8/0x174) [<c08a65a0>] (rt_mutex_slowlock_block.constprop.0) from [<c08a6708>] +(rt_mutex_slowlock.constprop.0+0xac/0x174) [<c08a6708>] (rt_mutex_slowlock.constprop.0) from [<c0374d60>] (fat_write_inode+0x34/0x54) [<c0374d60>] (fat_write_inode) from [<c0297304>] (__writeback_single_inode+0x354/0x3ec) [<c0297304>] (__writeback_single_inode) from [<c0297998>] (writeback_sb_inodes+0x250/0x45c) [<c0297998>] (writeback_sb_inodes) from [<c0297c20>] (__writeback_inodes_wb+0x7c/0xb8) [<c0297c20>] (__writeback_inodes_wb) from [<c0297f24>] (wb_writeback+0x2c8/0x2e4) [<c0297f24>] (wb_writeback) from [<c0298c40>] (wb_workfn+0x1a4/0x3e4) [<c0298c40>] (wb_workfn) from [<c0138ab8>] (process_one_work+0x1fc/0x32c) [<c0138ab8>] (process_one_work) from [<c0139120>] (worker_thread+0x22c/0x2d8) [<c0139120>] (worker_thread) from [<c013e6e0>] (kthread+0x16c/0x178) [<c013e6e0>] (kthread) from [<c01000fc>] (ret_from_fork+0x14/0x38) Exception stack(0xc10e3fb0 to 0xc10e3ff8) 3fa0: 00000000 00000000 00000000 00000000 3fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 3fe0: 00000000 00000000 00000000 00000000 00000013 00000000
INFO: task tar:2083 blocked for more than 491 seconds. Not tainted 5.15.49-rt46 #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:tar state:D stack: 0 pid: 2083 ppid: 2082 flags:0x00000000 [<c08a3a10>] (__schedule) from [<c08a3d84>] (schedule+0xdc/0x134) [<c08a3d84>] (schedule) from [<c08a41b0>] (io_schedule+0x14/0x24) [<c08a41b0>] (io_schedule) from [<c08a455c>] (bit_wait_io+0xc/0x30) [<c08a455c>] (bit_wait_io) from [<c08a441c>] (__wait_on_bit_lock+0x54/0xa8) [<c08a441c>] (__wait_on_bit_lock) from [<c08a44f4>] (out_of_line_wait_on_bit_lock+0x84/0xb0) [<c08a44f4>] (out_of_line_wait_on_bit_lock) from [<c0371fb0>] (fat_mirror_bhs+0xa0/0x144) [<c0371fb0>] (fat_mirror_bhs) from [<c0372a68>] (fat_alloc_clusters+0x138/0x2a4) [<c0372a68>] (fat_alloc_clusters) from [<c0370b14>] (fat_alloc_new_dir+0x34/0x250) [<c0370b14>] (fat_alloc_new_dir) from [<c03787c0>] (vfat_mkdir+0x58/0x148) [<c03787c0>] (vfat_mkdir) from [<c0277b60>] (vfs_mkdir+0x68/0x98) [<c0277b60>] (vfs_mkdir) from [<c027b484>] (do_mkdirat+0xb0/0xec) [<c027b484>] (do_mkdirat) from [<c0100060>] (ret_fast_syscall+0x0/0x1c) Exception stack(0xc2e1bfa8 to 0xc2e1bff0) bfa0: 01ee42f0 01ee4208 01ee42f0 000041ed 00000000 00004000 bfc0: 01ee42f0 01ee4208 00000000 00000027 01ee4302 00000004 000dcb00 01ee4190 bfe0: 000dc368 bed11924 0006d4b0 b6ebddfc
Here the kworker is waiting on msdos_sb_info::s_lock which is held by tar which is in turn waiting for a buffer which is locked waiting to be flushed, but this operation is plugged in the kworker.
The lock is a normal struct mutex, so tsk_is_pi_blocked() will always return false on !RT and thus the behaviour changes for RT.
It seems that the intent here is to skip blk_flush_plug() in the case where a non-preemptible lock (such as a spinlock) has been converted to a rtmutex on RT, which is the case covered by the SM_RTLOCK_WAIT schedule flag. But sched_submit_work() is only called from schedule() which is never called in this scenario, so the check can simply be deleted.
Looking at the history of the -rt patchset, in fact this change was present from v5.9.1-rt20 until being dropped in v5.13-rt1 as it was part of a larger patch [1] most of which was replaced by commit b4bfa3fcfe3b ("sched/core: Rework the __schedule() preempt argument").
As described in [1]:
The schedule process must distinguish between blocking on a regular sleeping lock (rwsem and mutex) and a RT-only sleeping lock (spinlock and rwlock): - rwsem and mutex must flush block requests (blk_schedule_flush_plug()) even if blocked on a lock. This can not deadlock because this also happens for non-RT. There should be a warning if the scheduling point is within a RCU read section.
- spinlock and rwlock must not flush block requests. This will deadlock if the callback attempts to acquire a lock which is already acquired. Similarly to being preempted, there should be no warning if the scheduling point is within a RCU read section.
and with the tsk_is_pi_blocked() in the scheduler path, we hit the first issue.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/p...
Signed-off-by: John Keeping john@metanate.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Steven Rostedt (Google) rostedt@goodmis.org Link: https://lkml.kernel.org/r/20220708162702.1758865-1-john@metanate.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/sched/rt.h | 8 -------- kernel/sched/core.c | 8 ++++++-- 2 files changed, 6 insertions(+), 10 deletions(-)
diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h index e5af028c08b4..994c25640e15 100644 --- a/include/linux/sched/rt.h +++ b/include/linux/sched/rt.h @@ -39,20 +39,12 @@ static inline struct task_struct *rt_mutex_get_top_task(struct task_struct *p) } extern void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task); extern void rt_mutex_adjust_pi(struct task_struct *p); -static inline bool tsk_is_pi_blocked(struct task_struct *tsk) -{ - return tsk->pi_blocked_on != NULL; -} #else static inline struct task_struct *rt_mutex_get_top_task(struct task_struct *task) { return NULL; } # define rt_mutex_adjust_pi(p) do { } while (0) -static inline bool tsk_is_pi_blocked(struct task_struct *tsk) -{ - return false; -} #endif
extern void normalize_rt_tasks(void); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index da0bf6fe9ecd..410b04decb90 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6500,8 +6500,12 @@ static inline void sched_submit_work(struct task_struct *tsk) io_wq_worker_sleeping(tsk); }
- if (tsk_is_pi_blocked(tsk)) - return; + /* + * spinlock and rwlock must not flush block requests. This will + * deadlock if the callback attempts to acquire a lock which is + * already acquired. + */ + SCHED_WARN_ON(current->__state & TASK_RTLOCK_WAIT);
/* * If we are going to sleep and we have plugged IO queued,