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PR/52301 CVS commit: src/sys/ufs/lfs
The following reply was made to PR kern/52301; it has been noted by GNATS.
From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
Subject: PR/52301 CVS commit: src/sys/ufs/lfs
Date: Sun, 23 Feb 2020 08:40:37 +0000
Module Name: src
Committed By: riastradh
Date: Sun Feb 23 08:40:37 UTC 2020
src/sys/ufs/lfs: lfs_extern.h lfs_segment.c lfs_subr.c
Break deadlock in PR kern/52301.
The lock order is lfs_writer -> lfs_seglock. The problem in 52301 is
that lfs_segwrite violates this lock order by sometimes doing
lfs_seglock -> lfs_writer, either (a) when doing a checkpoint or (b),
opportunistically, when there are no dirops pending. Both cases can
deadlock, because dirops sometimes take the seglock (lfs_truncate,
(a) There may be dirops pending, and they may be waiting for the
seglock, so we can't wait for them to complete while holding the
(b) The test for fs->lfs_dirops == 0 happens unlocked, and the state
may change by the time lfs_writer_enter acquires lfs_lock.
To resolve this in each case:
(a) Do lfs_writer_enter before lfs_seglock, since we will need it
unconditionally anyway. The worst performance impact of this should
be that some dirops get delayed a little bit.
(b) Create a new lfs_writer_tryenter to use at this point so that the
test for fs->lfs_dirops == 0 and the acquisition of lfs_writer happen
atomically under lfs_lock.
To generate a diff of this commit:
cvs rdiff -u -r1.115 -r1.116 src/sys/ufs/lfs/lfs_extern.h
cvs rdiff -u -r1.284 -r1.285 src/sys/ufs/lfs/lfs_segment.c
cvs rdiff -u -r1.98 -r1.99 src/sys/ufs/lfs/lfs_subr.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
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