On Wed, Oct 18 2017, Shaohua Li wrote:
> On Thu, Oct 19, 2017 at 12:17:16PM +1100, Neil Brown wrote:
>>
>> Most often mddev_suspend() is called with
>> reconfig_mutex held. Make this a requirement in
>> preparation a subsequent patch. Also require
>> reconfig_mutex to be held for mddev_resume(),
>> partly for symmetry and partly to guarantee
>> no races with incr/decr of mddev->suspend.
>>
>> Taking the mutex in r5c_disable_writeback_async() is
>> a little tricky as this is called from a work queue
>> via log->disable_writeback_work, and flush_work()
>> is called on that while holding ->reconfig_mutex.
>> If the work item hasn't run before flush_work()
>> is called, the work function will not be able to
>> get the mutex.
>>
>> So we use mddev_trylock() inside the wait_event() call, and have that
>> abort when conf->log is set to NULL, which happens before
>> flush_work() is called.
>> We wait in mddev->sb_wait and ensure this is woken
>> when any of the conditions change. This requires
>> waking mddev->sb_wait in mddev_unlock(). This is only
>> like to trigger extra wake_ups of threads that needn't
>> be woken when metadata is being written, and that
>> doesn't happen often enough that the cost would be
>> noticeable.
>>
>> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
>
> --snip--
>
> Applied the other patches in the series including the 'remove quiesce(2)' one.
Thanks.
>
>> index 0b7406ac8ce1..6a631dd21f0b 100644
>> --- a/drivers/md/raid5-cache.c
>> +++ b/drivers/md/raid5-cache.c
>> @@ -693,6 +693,8 @@ static void r5c_disable_writeback_async(struct work_struct *work)
>> struct r5l_log *log = container_of(work, struct r5l_log,
>> disable_writeback_work);
>> struct mddev *mddev = log->rdev->mddev;
>> + struct r5conf *conf = mddev->private;
>> + int locked = 0;
>>
>> if (log->r5c_journal_mode == R5C_JOURNAL_MODE_WRITE_THROUGH)
>> return;
>> @@ -701,11 +703,15 @@ static void r5c_disable_writeback_async(struct work_struct *work)
>>
>> /* wait superblock change before suspend */
>> wait_event(mddev->sb_wait,
>> - !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags));
>> -
>> - mddev_suspend(mddev);
>> - log->r5c_journal_mode = R5C_JOURNAL_MODE_WRITE_THROUGH;
>> - mddev_resume(mddev);
>> + conf->log == NULL ||
>> + (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) &&
>> + (locked = mddev_trylock(mddev))));
>> + if (locked) {
>> + mddev_suspend(mddev);
>> + log->r5c_journal_mode = R5C_JOURNAL_MODE_WRITE_THROUGH;
>> + mddev_resume(mddev);
>> + mddev_unlock(mddev);
>> + }
>
> For this one, my point is:
>
> wait_event(mddev->sb_wait, conf->log == NULL ||
> !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags));
> if (conf->log == NULL)
> return;
>
> mddev_suspend(mddev);
> log->r5c_journal_mode = R5C_JOURNAL_MODE_WRITE_THROUGH;
> mddev_resume(mddev);
>
> does it work?
The
lockdep_assert_held(&mddev->reconfig_mutex);
in mddev_suspend() will complain.
If you put an mddev_lock() call in there to stop the complaint, and if
the work item doesn't start before the reconfig_mutex is taken prior to
stopping the array, then r5l_exit_log() can deadlock at
flush_work(&log->disable_writeback_work);
Thanks,
NeilBrown
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