From ddd10c2fe20e7ca6d11ddf84f905edba080b26a7 Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Mon, 14 May 2018 06:34:34 -0700 Subject: [PATCH] xfs: avoid ABBA deadlock when scrubbing parent pointers 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. Therefore, convert all iolock attempts to use trylock; if that fails, they can use the existing mechanisms to back off and try again. The ABBA deadlock didn't happen with a non-repair scrub because the transactions don't reserve any space, but repair scrubs require reservation in order to update metadata. However, any other concurrent metadata update (e.g. directory create in the parent) could also induce this deadlock with the parent scrubber. Signed-off-by: Darrick J. Wong Reviewed-by: Brian Foster --- fs/xfs/scrub/common.c | 22 ++++++++++++++++++++++ fs/xfs/scrub/common.h | 1 + fs/xfs/scrub/parent.c | 16 ++++++++++++++-- 3 files changed, 37 insertions(+), 2 deletions(-) diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c index 62b33c9..518bff2 100644 --- a/fs/xfs/scrub/common.c +++ b/fs/xfs/scrub/common.c @@ -844,3 +844,25 @@ xfs_scrub_metadata_inode_forks( return error; } + +/* + * 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 5d78bb9..119d9b6 100644 --- a/fs/xfs/scrub/common.h +++ b/fs/xfs/scrub/common.h @@ -156,5 +156,6 @@ static inline bool xfs_scrub_skip_xref(struct xfs_scrub_metadata *sm) } int xfs_scrub_metadata_inode_forks(struct xfs_scrub_context *sc); +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 fc33680..77c6b22 100644 --- a/fs/xfs/scrub/parent.c +++ b/fs/xfs/scrub/parent.c @@ -214,7 +214,9 @@ 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); @@ -223,8 +225,10 @@ 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 @@ -326,5 +330,13 @@ 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; } -- 2.7.4