On 01/14/2018 03:42 PM, Coly Li wrote:
> 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.
>
> Changelog:
> v2: fix the race issue in v1 patch.
> v1: initial buggy fix.
>
> Signed-off-by: Coly Li <colyli@xxxxxxx>
> Cc: Michael Lyle <mlyle@xxxxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxx>
> Cc: Junhui Tang <tang.junhui@xxxxxxxxxx>
> ---
> drivers/md/bcache/writeback.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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