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Fix deadlock between zfs umount & snapentry_expire 

zfs umount -> zfsctl_destroy() takes the zfs_snapshot_lock as a
writer and calls zfsctl_snapshot_unmount_cancel(), which waits
for snapentry_expire() if present (when snap is automounted).
This snapentry_expire() itself then waits for zfs_snapshot_lock
as a reader, resulting in a deadlock.

The fix is to only hold the zfs_snapshot_lock over the tree
lookup and removal.  After a successful lookup the lock can
be dropped and zfs_snapentry_t will remain valid until the
reference taken by the lookup is released.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rohan Puri <rohan.puri15@gmail.com>
Closes #7751
Closes #7752
This commit is contained in:
Rohan Puri 2018-07-28 18:32:12 +05:30 committed by Brian Behlendorf
parent 492f64e941
commit fd7265c646
1 changed files with 5 additions and 6 deletions
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11
module/zfs/zfs_ctldir.c
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@ -355,8 +355,6 @@ snapentry_expire(void *data)
static void static void
zfsctl_snapshot_unmount_cancel(zfs_snapentry_t *se) zfsctl_snapshot_unmount_cancel(zfs_snapentry_t *se)
{ {
ASSERT(RW_LOCK_HELD(&zfs_snapshot_lock));
if (taskq_cancel_id(system_delay_taskq, se->se_taskqid) == 0) { if (taskq_cancel_id(system_delay_taskq, se->se_taskqid) == 0) {
se->se_taskqid = TASKQID_INVALID; se->se_taskqid = TASKQID_INVALID;
zfsctl_snapshot_rele(se); zfsctl_snapshot_rele(se);
@ -567,13 +565,14 @@ zfsctl_destroy(zfsvfs_t *zfsvfs)
uint64_t objsetid = dmu_objset_id(zfsvfs->z_os); uint64_t objsetid = dmu_objset_id(zfsvfs->z_os);
rw_enter(&zfs_snapshot_lock, RW_WRITER); rw_enter(&zfs_snapshot_lock, RW_WRITER);
if ((se = zfsctl_snapshot_find_by_objsetid(spa, objsetid)) se = zfsctl_snapshot_find_by_objsetid(spa, objsetid);
!= NULL) { if (se != NULL)
zfsctl_snapshot_unmount_cancel(se);
zfsctl_snapshot_remove(se); zfsctl_snapshot_remove(se);
rw_exit(&zfs_snapshot_lock);
if (se != NULL) {
zfsctl_snapshot_unmount_cancel(se);
zfsctl_snapshot_rele(se); zfsctl_snapshot_rele(se);
} }
rw_exit(&zfs_snapshot_lock);
} else if (zfsvfs->z_ctldir) { } else if (zfsvfs->z_ctldir) {
iput(zfsvfs->z_ctldir); iput(zfsvfs->z_ctldir);
zfsvfs->z_ctldir = NULL; zfsvfs->z_ctldir = NULL;