From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:49322 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752118AbeDSM4I (ORCPT ); Thu, 19 Apr 2018 08:56:08 -0400 Date: Thu, 19 Apr 2018 08:56:07 -0400 From: Brian Foster Subject: Re: [PATCH 11/11] xfs: avoid ABBA deadlock when scrubbing parent pointers Message-ID: <20180419125606.GE25844@bfoster.bfoster> References: <152401916729.11465.4212188839231900136.stgit@magnolia> <152401924456.11465.4870308714829350533.stgit@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <152401924456.11465.4870308714829350533.stgit@magnolia> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: "Darrick J. Wong" Cc: linux-xfs@vger.kernel.org On Tue, Apr 17, 2018 at 07:40:44PM -0700, Darrick J. Wong wrote: > From: Darrick J. Wong > > 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 > --- > 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@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html