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Revert commit 1ee159f4

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Fix lock order inversion with zvol_open() as it did not account
for use of zvols as vdevs. The latter use cases resulted in the
lock order inversion deadlocks that involved spa_namespace_lock
and bdev->bd_mutex.

Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #6065
Issue #6134
This commit is contained in:
Boris Protopopov 2017-05-11 16:40:33 -04:00 committed by Brian Behlendorf
parent 3d6da72d18
commit 07783588bc
1 changed files with 29 additions and 2 deletions
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31
module/zfs/zvol.c
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@ -1150,12 +1150,36 @@ static int
zvol_first_open(zvol_state_t *zv)
{
objset_t *os;
int error;
int error, locked = 0;
/*
* In all other cases the spa_namespace_lock is taken before the
* bdev->bd_mutex lock. But in this case the Linux __blkdev_get()
* function calls fops->open() with the bdev->bd_mutex lock held.
* This deadlock can be easily observed with zvols used as vdevs.
*
* To avoid a potential lock inversion deadlock we preemptively
* try to take the spa_namespace_lock(). Normally it will not
* be contended and this is safe because spa_open_common() handles
* the case where the caller already holds the spa_namespace_lock.
*
* When it is contended we risk a lock inversion if we were to
* block waiting for the lock. Luckily, the __blkdev_get()
* function allows us to return -ERESTARTSYS which will result in
* bdev->bd_mutex being dropped, reacquired, and fops->open() being
* called again. This process can be repeated safely until both
* locks are acquired.
*/
if (!mutex_owned(&spa_namespace_lock)) {
locked = mutex_tryenter(&spa_namespace_lock);
if (!locked)
return (-SET_ERROR(ERESTARTSYS));
}
/* lie and say we're read-only */
error = dmu_objset_own(zv->zv_name, DMU_OST_ZVOL, 1, zv, &os);
if (error)
return (SET_ERROR(-error));
goto out_mutex;
zv->zv_objset = os;
@ -1166,6 +1190,9 @@ zvol_first_open(zvol_state_t *zv)
zv->zv_objset = NULL;
}
out_mutex:
if (locked)
mutex_exit(&spa_namespace_lock);
return (SET_ERROR(-error));
}