linux-kernel - [PATCH AUTOSEL for 3.18 096/101] bcache: properly set task state in bch_writeback

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Message-ID: <20180409004042.164920-46-alexander.levin@microsoft.com>
Date:   Mon, 9 Apr 2018 00:41:52 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Coly Li <colyli@...e.de>, Michael Lyle <mlyle@...e.org>,
        Junhui Tang <tang.junhui@....com.cn>,
        Jens Axboe <axboe@...nel.dk>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 3.18 096/101] bcache: properly set task state in
 bch_writeback_thread()

From: Coly Li <colyli@...e.de>

[ Upstream commit 99361bbf26337186f02561109c17a4c4b1a7536a ]

Kernel thread routine bch_writeback_thread() has the following code block,

447         down_write(&dc->writeback_lock);
448~450     if (check conditions) {
451                 up_write(&dc->writeback_lock);
452                 set_current_state(TASK_INTERRUPTIBLE);
453
454                 if (kthread_should_stop())
455                         return 0;
456
457                 schedule();
458                 continue;
459         }

If condition check is true, its task state is set to TASK_INTERRUPTIBLE
and call schedule() to wait for others to wake up it.

There are 2 issues in current code,
1, Task state is set to TASK_INTERRUPTIBLE after the condition checks, if
   another process changes the condition and call wake_up_process(dc->
   writeback_thread), then at line 452 task state is set back to
   TASK_INTERRUPTIBLE, the writeback kernel thread will lose a chance to be
   waken up.
2, At line 454 if kthread_should_stop() is true, writeback kernel thread
   will return to kernel/kthread.c:kthread() with TASK_INTERRUPTIBLE and
   call do_exit(). It is not good to enter do_exit() with task state
   TASK_INTERRUPTIBLE, in following code path might_sleep() is called and a
   warning message is reported by __might_sleep(): "WARNING: do not call
   blocking ops when !TASK_RUNNING; state=1 set at [xxxx]".

For the first issue, task state should be set before condition checks.
Ineed because dc->writeback_lock is required when modifying all the
conditions, calling set_current_state() inside code block where dc->
writeback_lock is hold is safe. But this is quite implicit, so I still move
set_current_state() before all the condition checks.

For the second issue, frankley speaking it does not hurt when kernel thread
exits with TASK_INTERRUPTIBLE state, but this warning message scares users,
makes them feel there might be something risky with bcache and hurt their
data.  Setting task state to TASK_RUNNING before returning fixes this
problem.

In alloc.c:allocator_wait(), there is also a similar issue, and is also
fixed in this patch.

Changelog:
v3: merge two similar fixes into one patch
v2: fix the race issue in v1 patch.
v1: initial buggy fix.

Signed-off-by: Coly Li <colyli@...e.de>
Reviewed-by: Hannes Reinecke <hare@...e.de>
Reviewed-by: Michael Lyle <mlyle@...e.org>
Cc: Michael Lyle <mlyle@...e.org>
Cc: Junhui Tang <tang.junhui@....com.cn>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 drivers/md/bcache/alloc.c     | 4 +++-
 drivers/md/bcache/writeback.c | 7 +++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index ea47980949ef..c1da2321bf26 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -285,8 +285,10 @@ do {									\
 			break;						\
 									\
 		mutex_unlock(&(ca)->set->bucket_lock);			\
-		if (kthread_should_stop())				\
+		if (kthread_should_stop()) {				\
+			set_current_state(TASK_RUNNING);		\
 			return 0;					\
+		}							\
 									\
 		try_to_freeze();					\
 		schedule();						\
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index b0667b321a3f..50726f12a7c3 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -425,19 +425,22 @@ static int bch_writeback_thread(void *arg)

 	while (!kthread_should_stop()) {
 		down_write(&dc->writeback_lock);
+		set_current_state(TASK_INTERRUPTIBLE);
 		if (!atomic_read(&dc->has_dirty) ||
 		    (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
 		     !dc->writeback_running)) {
 			up_write(&dc->writeback_lock);
-			set_current_state(TASK_INTERRUPTIBLE);

-			if (kthread_should_stop())
+			if (kthread_should_stop()) {
+				set_current_state(TASK_RUNNING);
 				return 0;
+			}

 			try_to_freeze();
 			schedule();
 			continue;
 		}
+		set_current_state(TASK_RUNNING);

 		searched_full_index = refill_dirty(dc);

-- 
2.15.1