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Fix zrele race in zrele_async that can cause hang

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There is a race condition in zfs_zrele_async when we are checking if 
we would be the one to evict an inode. This can lead to a txg sync 
deadlock.

Instead of calling into iput directly, we attempt to perform the atomic 
decrement ourselves, unless that would set the i_count value to zero. 
In that case, we dispatch a call to iput to run later, to prevent a 
deadlock from occurring.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #11527 
Closes #11530
This commit is contained in:
Paul Dagnelie 2021-01-27 21:29:58 -08:00 committed by GitHub
parent b8e6401b79
commit 2921ad6cba
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1 changed files with 22 additions and 12 deletions
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34
module/os/linux/zfs/zfs_vnops_os.c
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@ -87,15 +87,18 @@
* must be checked with ZFS_VERIFY_ZP(zp). Both of these macros * must be checked with ZFS_VERIFY_ZP(zp). Both of these macros
* can return EIO from the calling function. * can return EIO from the calling function.
* *
* (2) zrele() should always be the last thing except for zil_commit() * (2) zrele() should always be the last thing except for zil_commit() (if
* (if necessary) and ZFS_EXIT(). This is for 3 reasons: * necessary) and ZFS_EXIT(). This is for 3 reasons: First, if it's the
* First, if it's the last reference, the vnode/znode * last reference, the vnode/znode can be freed, so the zp may point to
* can be freed, so the zp may point to freed memory. Second, the last * freed memory. Second, the last reference will call zfs_zinactive(),
* reference will call zfs_zinactive(), which may induce a lot of work -- * which may induce a lot of work -- pushing cached pages (which acquires
* pushing cached pages (which acquires range locks) and syncing out * range locks) and syncing out cached atime changes. Third,
* cached atime changes. Third, zfs_zinactive() may require a new tx, * zfs_zinactive() may require a new tx, which could deadlock the system
* which could deadlock the system if you were already holding one. * if you were already holding one. This deadlock occurs because the tx
* If you must call zrele() within a tx then use zfs_zrele_async(). * currently being operated on prevents a txg from syncing, which
* prevents the new tx from progressing, resulting in a deadlock. If you
* must call zrele() within a tx, use zfs_zrele_async(). Note that iput()
* is a synonym for zrele().
* *
* (3) All range locks must be grabbed before calling dmu_tx_assign(), * (3) All range locks must be grabbed before calling dmu_tx_assign(),
* as they can span dmu_tx_assign() calls. * as they can span dmu_tx_assign() calls.
@ -398,11 +401,18 @@ zfs_zrele_async(znode_t *zp)
ASSERT(atomic_read(&ip->i_count) > 0); ASSERT(atomic_read(&ip->i_count) > 0);
ASSERT(os != NULL); ASSERT(os != NULL);
if (atomic_read(&ip->i_count) == 1) /*
* If decrementing the count would put us at 0, we can't do it inline
* here, because that would be synchronous. Instead, dispatch an iput
* to run later.
*
* For more information on the dangers of a synchronous iput, see the
* header comment of this file.
*/
if (!atomic_add_unless(&ip->i_count, -1, 1)) {
VERIFY(taskq_dispatch(dsl_pool_zrele_taskq(dmu_objset_pool(os)), VERIFY(taskq_dispatch(dsl_pool_zrele_taskq(dmu_objset_pool(os)),
(task_func_t *)iput, ip, TQ_SLEEP) != TASKQID_INVALID); (task_func_t *)iput, ip, TQ_SLEEP) != TASKQID_INVALID);
else }
zrele(zp);
} }