Re: [PATCH 2/8] xfs: fix missed wakeup on l_flush_wait

Darrick J. Wong
2019-09-05T15:21:41Z

On Thu, Sep 05, 2019 at 06:47:11PM +1000, Dave Chinner wrote:
> From: Rik van Riel <riel@surriel.com>
>
> The code in xlog_wait uses the spinlock to make adding the task to
> the wait queue, and setting the task state to UNINTERRUPTIBLE atomic
> with respect to the waker.
>
> Doing the wakeup after releasing the spinlock opens up the following
> race condition:
>
> Task 1 task 2
> add task to wait queue
> wake up task
> set task state to UNINTERRUPTIBLE
>
> This issue was found through code inspection as a result of kworkers
> being observed stuck in UNINTERRUPTIBLE state with an empty
> wait queue. It is rare and largely unreproducable.
>
> Simply moving the spin_unlock to after the wake_up_all results
> in the waker not being able to see a task on the waitqueue before
> it has set its state to UNINTERRUPTIBLE.
>
> This bug dates back to the conversion of this code to generic
> waitqueue infrastructure from a counting semaphore back in 2008
> which didn't place the wakeups consistently w.r.t. to the relevant
> spin locks.
>
> [dchinner: Also fix a similar issue in the shutdown path on
> xc_commit_wait. Update commit log with more details of the issue.]
>
> Fixes: d748c62367eb ("[XFS] Convert l_flushsema to a sv_t")
> Reported-by: Chris Mason <clm@fb.com>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Looks ok to me,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> fs/xfs/xfs_log.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 5e21450fb8f5..8380ed065608 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2644,7 +2644,6 @@ xlog_state_do_callback(
> int funcdidcallbacks; /* flag: function did callbacks */
> int repeats; /* for issuing console warnings if
> * looping too many times */
> - int wake = 0;
>
> spin_lock(&log->l_icloglock);
> first_iclog = iclog = log->l_iclog;
> @@ -2840,11 +2839,9 @@ xlog_state_do_callback(
> #endif
>
> if (log->l_iclog->ic_state & (XLOG_STATE_ACTIVE|XLOG_STATE_IOERROR))
> - wake = 1;
> - spin_unlock(&log->l_icloglock);
> -
> - if (wake)
> wake_up_all(&log->l_flush_wait);
> +
> + spin_unlock(&log->l_icloglock);
> }
>
>
> @@ -3944,7 +3941,9 @@ xfs_log_force_umount(
> * item committed callback functions will do this again under lock to
> * avoid races.
> */
> + spin_lock(&log->l_cilp->xc_push_lock);
> wake_up_all(&log->l_cilp->xc_commit_wait);
> + spin_unlock(&log->l_cilp->xc_push_lock);
> xlog_state_do_callback(log, true, NULL);
>
> #ifdef XFSERRORDEBUG
> --
> 2.23.0.rc1
>