Fix zpool on zvol lock inversion deadlock

In all but one case the spa_namespace_lock is taken before the
bdev->bd_mutex lock.  But Linux __blkdev_get() function calls
fops->open() with the bdev->bd_mutex lock held and we must
somehow still safely acquire the spa_namespace_lock.

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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #612

diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c
index 7a448f1..d4d533f 100644 (file)
--- a/module/zfs/zvol.c
+++ b/module/zfs/zvol.c
@@ -891,11 +891,39 @@ zvol_first_open(zvol_state_t *zv)
 {
        objset_t *os;
        uint64_t volsize;
+       int locked = 0;
        int error;
        uint64_t ro;
 
+       /*
+        * 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.
+        *
+        * 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 (-ERESTARTSYS);
+       }
+
        /* lie and say we're read-only */
        error = dmu_objset_own(zv->zv_name, DMU_OST_ZVOL, 1, zvol_tag, &os);
+
+       if (locked)
+               mutex_exit(&spa_namespace_lock);
+
        if (error)
                return (-error);