On Tue, Apr 17, 2018 at 07:40:44PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> In normal operation, the XFS convention is to take an inode's iolock
> and then allocate a transaction. However, when scrubbing parent inodes
> this is inverted -- we allocated the transaction to do the scrub, and
> now we're trying to grab the parent's iolock. This can lead to ABBA
> deadlocks: some thread grabbed the parent's iolock and is waiting for
> space for a transaction while our parent scrubber is sitting on a
> transaction trying to get the parent's iolock.
>
Is that really an issue if the scrub transaction doesn't acquire a log
reservation (or does it in certain circumstances)..?
Brian
> Therefore, convert all iolock attempts to use trylock; if that fails,
> they can use the existing mechanisms to back off and try again.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> fs/xfs/scrub/common.c | 22 ++++++++++++++++++++++
> fs/xfs/scrub/common.h | 2 ++
> fs/xfs/scrub/parent.c | 16 ++++++++++++++--
> 3 files changed, 38 insertions(+), 2 deletions(-)
>
>
> diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
> index f5e281a..93f9e7d 100644
> --- a/fs/xfs/scrub/common.c
> +++ b/fs/xfs/scrub/common.c
> @@ -787,3 +787,25 @@ xfs_scrub_buffer_recheck(
> sc->sm->sm_flags |= XFS_SCRUB_OFLAG_CORRUPT;
> trace_xfs_scrub_block_error(sc, bp->b_bn, fa);
> }
> +
> +/*
> + * Try to lock an inode in violation of the usual locking order rules. For
> + * example, trying to get the IOLOCK while in transaction context, or just
> + * plain breaking AG-order or inode-order inode locking rules. Either way,
> + * the only way to avoid an ABBA deadlock is to use trylock and back off if
> + * we can't.
> + */
> +int
> +xfs_scrub_ilock_inverted(
> + struct xfs_inode *ip,
> + uint lock_mode)
> +{
> + int i;
> +
> + for (i = 0; i < 20; i++) {
> + if (xfs_ilock_nowait(ip, lock_mode))
> + return 0;
> + delay(1);
> + }
> + return -EDEADLOCK;
> +}
> diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
> index 8296873..191c369 100644
> --- a/fs/xfs/scrub/common.h
> +++ b/fs/xfs/scrub/common.h
> @@ -151,4 +151,6 @@ static inline bool xfs_scrub_found_corruption(struct xfs_scrub_metadata *sm)
> XFS_SCRUB_OFLAG_XCORRUPT);
> }
>
> +int xfs_scrub_ilock_inverted(struct xfs_inode *ip, uint lock_mode);
> +
> #endif /* __XFS_SCRUB_COMMON_H__ */
> diff --git a/fs/xfs/scrub/parent.c b/fs/xfs/scrub/parent.c
> index 1fb88c1..19cd54d 100644
> --- a/fs/xfs/scrub/parent.c
> +++ b/fs/xfs/scrub/parent.c
> @@ -211,7 +211,9 @@ xfs_scrub_parent_validate(
> */
> xfs_iunlock(sc->ip, sc->ilock_flags);
> sc->ilock_flags = 0;
> - xfs_ilock(dp, XFS_IOLOCK_SHARED);
> + error = xfs_scrub_ilock_inverted(dp, XFS_IOLOCK_SHARED);
> + if (error)
> + goto out_rele;
>
> /* Go looking for our dentry. */
> error = xfs_scrub_parent_count_parent_dentries(sc, dp, &nlink);
> @@ -220,8 +222,10 @@ xfs_scrub_parent_validate(
>
> /* Drop the parent lock, relock this inode. */
> xfs_iunlock(dp, XFS_IOLOCK_SHARED);
> + error = xfs_scrub_ilock_inverted(sc->ip, XFS_IOLOCK_EXCL);
> + if (error)
> + goto out_rele;
> sc->ilock_flags = XFS_IOLOCK_EXCL;
> - xfs_ilock(sc->ip, sc->ilock_flags);
>
> /*
> * If we're an unlinked directory, the parent /won't/ have a link
> @@ -323,5 +327,13 @@ xfs_scrub_parent(
> if (try_again && tries == 20)
> xfs_scrub_set_incomplete(sc);
> out:
> + /*
> + * If we failed to lock the parent inode even after a retry, just mark
> + * this scrub incomplete and return.
> + */
> + if (sc->try_harder && error == -EDEADLOCK) {
> + error = 0;
> + xfs_scrub_set_incomplete(sc);
> + }
> return error;
> }
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
