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CVS commit: src/sys/ufs/lfs

Module Name:    src
Committed By:   riastradh
Date:           Sun Feb 23 08:40:37 UTC 2020

Modified Files:
        src/sys/ufs/lfs: lfs_extern.h lfs_segment.c lfs_subr.c

Log Message:
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,
lfs_valloc, lfs_vfree):

(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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