From 65d56083b4617a4cade0cff68cbbaf68114169d6 Mon Sep 17 00:00:00 2001 From: Brian Behlendorf Date: Wed, 19 Dec 2012 13:51:44 -0800 Subject: [PATCH] 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 Signed-off-by: Jorgen Lundman Closes #612 --- module/zfs/zvol.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c index 7a448f194..d4d533f02 100644 --- 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); -- 2.40.0