On Thu, May 24, 2012 at 12:06:42PM -0400, Brian Foster wrote:
> xfsaild idle mode logic currently leads to a couple hangs:
>
> 1.) If xfsaild is rescheduled in during an incremental scan
> (i.e., tout != 0) and the target has been updated since
> the previous run, we can hit the new target and go into
> idle mode with a still populated ail.
> 2.) A wake up is only issued when the target is pushed forward.
> The wake up can race with xfsaild if it is currently in the
> process of entering idle mode, causing future wake up
> events to be lost.
>
> Both hangs are reproducible by running xfstests 273 in a loop.
> Modify xfsaild to enter idle mode only when the ail is empty
> and the push target has not been moved forward since the last
> push.
>
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
>
> This is a lightly tested version against the xfs tree. I'll be more
> heavily testing a version based on my upstream reproducer tree over the
> next few days followed by similar testing on this patch if all goes
> well. Sending in advance for review.
>
> fs/xfs/xfs_trans_ail.c | 29 ++++++++++++++++++++++++++---
> fs/xfs/xfs_trans_priv.h | 1 +
> 2 files changed, 27 insertions(+), 3 deletions(-)
Looks OK but some minor formatting nits.
> @@ -527,8 +532,26 @@ xfsaild(
> __set_current_state(TASK_KILLABLE);
> else
> __set_current_state(TASK_INTERRUPTIBLE);
> - schedule_timeout(tout ?
> - msecs_to_jiffies(tout) : MAX_SCHEDULE_TIMEOUT);
> +
> + spin_lock(&ailp->xa_lock);
> +
> + /* barrier matches the xa_target update in xfs_ail_push() */
> + smp_rmb();
> + if (!xfs_ail_min(ailp) && (ailp->xa_target == ailp->xa_target_prev)) {
> + /* the ail is empty and no change to the push target - idle */
I much prefer comments above the if() statement - the natural order
of reading something is top down - explain why, then do. iAlso,
there is no reason to write comments in abbreviated english - make
them verbose so that when you come back to this code in 2 years time
you can immediately understand why the code is like this from the
comment. So this:
/*
* Idle if the ail is empty and we are not racing
* with a target update. The barrier matches the
* xa_target update in xfs_ail_push().
*/
smp_rmb();
if (!xfs_ail_min(ailp) && (ailp->xa_target == ailp->xa_target_prev)) {
> + spin_unlock(&ailp->xa_lock);
> + schedule();
> + tout = 0;
> + continue;
> + }
> + spin_unlock(&ailp->xa_lock);
> +
> + if (tout) {
> + /* more work to do soon */
> + schedule_timeout(msecs_to_jiffies(tout));
> + }
And here I think this comment is redundant, because the code
is self documenting - if we have a timeout set, sleep for that
timeout, otherwise continue and do more work....
if (tout)
schedule_timeout(msecs_to_jiffies(tout));
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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