On Thu, Oct 19, 2017 at 05:29:22PM +1100, Neil Brown wrote:
> 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);
Ok, got it now. But really don't like this patch. The mddev_unlock is strange,
r5c_disable_writeback_async could skip disabling writeback too. Could we add a
new callback like .prepare_free, and flush workqueue there. After we drop the
reconfig_mutex in do_md_stop, we call the prepare_free. We can probably set a
flag, so later r5c_disable_writeback_async will bail out doing nothing. I think
this should work, right?
Thanks,
Shaohua
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