• Coly Li's avatar
    bcache: properly set task state in bch_writeback_thread() · 99361bbf
    Coly Li authored
    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: default avatarColy Li <colyli@suse.de>
    Reviewed-by: default avatarHannes Reinecke <hare@suse.de>
    Reviewed-by: default avatarMichael Lyle <mlyle@lyle.org>
    Cc: Michael Lyle <mlyle@lyle.org>
    Cc: Junhui Tang <tang.junhui@zte.com.cn>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    99361bbf
alloc.c 18.6 KB