4.14-stable review patch. If anyone has any objections, please let me know.
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From: Coly Li colyli@suse.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@suse.de Reviewed-by: Hannes Reinecke hare@suse.de Reviewed-by: Michael Lyle mlyle@lyle.org Cc: Michael Lyle mlyle@lyle.org Cc: Junhui Tang tang.junhui@zte.com.cn Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/md/bcache/alloc.c | 4 +++- drivers/md/bcache/writeback.c | 7 +++++-- 2 files changed, 8 insertions(+), 3 deletions(-)
--- a/drivers/md/bcache/alloc.c +++ b/drivers/md/bcache/alloc.c @@ -287,8 +287,10 @@ do { \ break; \ \ mutex_unlock(&(ca)->set->bucket_lock); \ - if (kthread_should_stop()) \ + if (kthread_should_stop()) { \ + set_current_state(TASK_RUNNING); \ return 0; \ + } \ \ schedule(); \ mutex_lock(&(ca)->set->bucket_lock); \ --- a/drivers/md/bcache/writeback.c +++ b/drivers/md/bcache/writeback.c @@ -420,18 +420,21 @@ static int bch_writeback_thread(void *ar
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; + }
schedule(); continue; } + set_current_state(TASK_RUNNING);
searched_full_index = refill_dirty(dc);