Linux 5.13 compat: retry zvol_open() when contended

Due to a possible lock inversion the zvol open call path on Linux
needs to be able to retry in the case where the spa_namespace_lock
cannot be acquired.

For Linux 5.12 an older kernel this was accomplished by returning
-ERESTARTSYS from zvol_open() to request that blkdev_get() drop
the bdev->bd_mutex lock, reaquire it, then call the open callback
again.  However, as of the 5.13 kernel this behavior was removed.

Therefore, for 5.12 and older kernels we preserved the existing
retry logic, but for 5.13 and newer kernels we retry internally in
zvol_open().  This should always succeed except in the case where
a pool's vdev are layed on zvols, in which case it may fail.  To
handle this case vdev_disk_open() has been updated to retry when
opening a device when -ERESTARTSYS is returned.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Tony Nguyen <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Issue #12301
Closes #12759

1 files changed, 5 insertions, 33 deletions

diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c
index e4b641989..721a7b4b8 100644
--- a/module/zfs/zvol.c
+++ b/module/zfs/zvol.c
@@ -909,54 +909,26 @@ int
 zvol_first_open(zvol_state_t *zv, boolean_t readonly)
 {
 	objset_t *os;
-	int error, locked = 0;
-	boolean_t ro;
+	int error;
 
 	ASSERT(RW_READ_HELD(&zv->zv_suspend_lock));
 	ASSERT(MUTEX_HELD(&zv->zv_state_lock));
+	ASSERT(mutex_owned(&spa_namespace_lock));
 
-	/*
-	 * 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(EINTR));
-	}
-
-	ro = (readonly || (strchr(zv->zv_name, '@') != NULL));
+	boolean_t ro = (readonly || (strchr(zv->zv_name, '@') != NULL));
 	error = dmu_objset_own(zv->zv_name, DMU_OST_ZVOL, ro, B_TRUE, zv, &os);
 	if (error)
-		goto out_mutex;
+		return (SET_ERROR(error));
 
 	zv->zv_objset = os;
 
 	error = zvol_setup_zv(zv);
-
 	if (error) {
 		dmu_objset_disown(os, 1, zv);
 		zv->zv_objset = NULL;
 	}
 
-out_mutex:
-	if (locked)
-		mutex_exit(&spa_namespace_lock);
-	return (SET_ERROR(error));
+	return (error);
 }
 
 void