[PATCH v3] sched/core: Always flush pending blk_plug

* [PATCH v3] sched/core: Always flush pending blk_plug
@ 2022-07-08 16:27 John Keeping
  2022-07-09 16:19 ` Paul Gortmaker
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: John Keeping @ 2022-07-08 16:27 UTC (permalink / raw)
  To: linux-kernel
  Cc: linux-rt-users, John Keeping, Sebastian Andrzej Siewior,
	Peter Zijlstra, Thomas Gleixner, Steven Rostedt, Ingo Molnar,
	Juri Lelli, Vincent Guittot, Dietmar Eggemann, Ben Segall,
	Mel Gorman, Daniel Bristot de Oliveira, Valentin Schneider

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/patches/0022-locking-rtmutex-Use-custom-scheduling-function-for-s.patch?h=linux-5.10.y-rt-patches

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: John Keeping <john@metanate.com>
---
v3:
- Add SCHED_WARN_ON(current->__state & TASK_RTLOCK_WAIT) as suggested by
  Peter
v2:
- Add Steven's R-b and update the commit message with his suggested
  quote from [1]

 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 e5af028c08b49..994c25640e156 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 1d4660a1915b3..71d6385ece83f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6578,8 +6578,12 @@ 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,
-- 
2.37.0


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