MFC r286689: 5981 Deadlock in dmu_objset_find_dp

illumos/illumos-gate@1d3f896f5469c69c1339890ec3d68e9feddb0343

https://www.illumos.org/issues/5981
  When dmu_objset_find_dp gets called with a read lock held, it fans out
  the work to the task queue. Each task in turn acquires its own read
  lock before calling the callback. If during this process anyone tries
  to a acquire a write lock, it will stall all read lock requests.Thus
  the tasks will never finish, the read lock of the caller will never
  get freed and the write lock never acquired.  deadlock.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Arne Jansen <jansen@webgods.de>

1 files changed, 9 insertions, 1 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_objset.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_objset.c
index 646968e..9ad18f0 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_objset.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_objset.c
@@ -1746,7 +1746,15 @@ dmu_objset_find_dp_cb(void *arg)
 	dmu_objset_find_ctx_t *dcp = arg;
 	dsl_pool_t *dp = dcp->dc_dp;
 
-	dsl_pool_config_enter(dp, FTAG);
+	/*
+	 * We need to get a pool_config_lock here, as there are several
+	 * asssert(pool_config_held) down the stack. Getting a lock via
+	 * dsl_pool_config_enter is risky, as it might be stalled by a
+	 * pending writer. This would deadlock, as the write lock can
+	 * only be granted when our parent thread gives up the lock.
+	 * The _prio interface gives us priority over a pending writer.
+	 */
+	dsl_pool_config_enter_prio(dp, FTAG);
 
 	dmu_objset_find_dp_impl(dcp);