Fix dnode_hold_impl() soft lockup

Soft lockups could happen when multiple threads trying
to get zrl on the same dnode handle in order to allocate
and initialize the dnode marked as DN_SLOT_ALLOCATED.

Don't loop from beginning when we can't get zrl, otherwise
we would increase the zrl refcount and nobody can actually
lock it.

Reviewed by: Tom Caputi <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Li Dongyang <[email protected]>
Closes #8433 

2 files changed, 53 insertions, 56 deletions

diff --git a/include/sys/zfs_context.h b/include/sys/zfs_context.h
index 11a32bb31..260b8a458 100644
--- a/include/sys/zfs_context.h
+++ b/include/sys/zfs_context.h
@@ -236,6 +236,7 @@ extern kthread_t *zk_thread_create(void (*func)(void *), void *arg,
 
 #define	kpreempt_disable()	((void)0)
 #define	kpreempt_enable()	((void)0)
+#define	cond_resched()		sched_yield()
 
 /*
  * Mutexes
diff --git a/module/zfs/dnode.c b/module/zfs/dnode.c
index b7a7f90cf..9a19ddabb 100644
--- a/module/zfs/dnode.c
+++ b/module/zfs/dnode.c
@@ -1153,8 +1153,10 @@ dnode_free_interior_slots(dnode_t *dn)
 
 	ASSERT3S(idx + slots, <=, DNODES_PER_BLOCK);
 
-	while (!dnode_slots_tryenter(children, idx, slots))
+	while (!dnode_slots_tryenter(children, idx, slots)) {
 		DNODE_STAT_BUMP(dnode_free_interior_lock_retry);
+		cond_resched();
+	}
 
 	dnode_set_slots(children, idx, slots, DN_SLOT_FREE);
 	dnode_slots_rele(children, idx, slots);
@@ -1401,34 +1403,30 @@ dnode_hold_impl(objset_t *os, uint64_t object, int flag, int slots,
 	}
 
 	ASSERT(dnc->dnc_count == epb);
-	dn = DN_SLOT_UNINIT;
 
 	if (flag & DNODE_MUST_BE_ALLOCATED) {
 		slots = 1;
 
-		while (dn == DN_SLOT_UNINIT) {
-			dnode_slots_hold(dnc, idx, slots);
-			dnh = &dnc->dnc_children[idx];
-
-			if (DN_SLOT_IS_PTR(dnh->dnh_dnode)) {
-				dn = dnh->dnh_dnode;
-				break;
-			} else if (dnh->dnh_dnode == DN_SLOT_INTERIOR) {
-				DNODE_STAT_BUMP(dnode_hold_alloc_interior);
-				dnode_slots_rele(dnc, idx, slots);
-				dbuf_rele(db, FTAG);
-				return (SET_ERROR(EEXIST));
-			} else if (dnh->dnh_dnode != DN_SLOT_ALLOCATED) {
-				DNODE_STAT_BUMP(dnode_hold_alloc_misses);
-				dnode_slots_rele(dnc, idx, slots);
-				dbuf_rele(db, FTAG);
-				return (SET_ERROR(ENOENT));
-			}
+		dnode_slots_hold(dnc, idx, slots);
+		dnh = &dnc->dnc_children[idx];
 
+		if (DN_SLOT_IS_PTR(dnh->dnh_dnode)) {
+			dn = dnh->dnh_dnode;
+		} else if (dnh->dnh_dnode == DN_SLOT_INTERIOR) {
+			DNODE_STAT_BUMP(dnode_hold_alloc_interior);
 			dnode_slots_rele(dnc, idx, slots);
-			if (!dnode_slots_tryenter(dnc, idx, slots)) {
+			dbuf_rele(db, FTAG);
+			return (SET_ERROR(EEXIST));
+		} else if (dnh->dnh_dnode != DN_SLOT_ALLOCATED) {
+			DNODE_STAT_BUMP(dnode_hold_alloc_misses);
+			dnode_slots_rele(dnc, idx, slots);
+			dbuf_rele(db, FTAG);
+			return (SET_ERROR(ENOENT));
+		} else {
+			dnode_slots_rele(dnc, idx, slots);
+			while (!dnode_slots_tryenter(dnc, idx, slots)) {
 				DNODE_STAT_BUMP(dnode_hold_alloc_lock_retry);
-				continue;
+				cond_resched();
 			}
 
 			/*
@@ -1463,45 +1461,43 @@ dnode_hold_impl(objset_t *os, uint64_t object, int flag, int slots,
 			return (SET_ERROR(ENOSPC));
 		}
 
-		while (dn == DN_SLOT_UNINIT) {
-			dnode_slots_hold(dnc, idx, slots);
-
-			if (!dnode_check_slots_free(dnc, idx, slots)) {
-				DNODE_STAT_BUMP(dnode_hold_free_misses);
-				dnode_slots_rele(dnc, idx, slots);
-				dbuf_rele(db, FTAG);
-				return (SET_ERROR(ENOSPC));
-			}
+		dnode_slots_hold(dnc, idx, slots);
 
+		if (!dnode_check_slots_free(dnc, idx, slots)) {
+			DNODE_STAT_BUMP(dnode_hold_free_misses);
 			dnode_slots_rele(dnc, idx, slots);
-			if (!dnode_slots_tryenter(dnc, idx, slots)) {
-				DNODE_STAT_BUMP(dnode_hold_free_lock_retry);
-				continue;
-			}
+			dbuf_rele(db, FTAG);
+			return (SET_ERROR(ENOSPC));
+		}
 
-			if (!dnode_check_slots_free(dnc, idx, slots)) {
-				DNODE_STAT_BUMP(dnode_hold_free_lock_misses);
-				dnode_slots_rele(dnc, idx, slots);
-				dbuf_rele(db, FTAG);
-				return (SET_ERROR(ENOSPC));
-			}
+		dnode_slots_rele(dnc, idx, slots);
+		while (!dnode_slots_tryenter(dnc, idx, slots)) {
+			DNODE_STAT_BUMP(dnode_hold_free_lock_retry);
+			cond_resched();
+		}
 
-			/*
-			 * Allocated but otherwise free dnodes which would
-			 * be in the interior of a multi-slot dnodes need
-			 * to be freed.  Single slot dnodes can be safely
-			 * re-purposed as a performance optimization.
-			 */
-			if (slots > 1)
-				dnode_reclaim_slots(dnc, idx + 1, slots - 1);
+		if (!dnode_check_slots_free(dnc, idx, slots)) {
+			DNODE_STAT_BUMP(dnode_hold_free_lock_misses);
+			dnode_slots_rele(dnc, idx, slots);
+			dbuf_rele(db, FTAG);
+			return (SET_ERROR(ENOSPC));
+		}
 
-			dnh = &dnc->dnc_children[idx];
-			if (DN_SLOT_IS_PTR(dnh->dnh_dnode)) {
-				dn = dnh->dnh_dnode;
-			} else {
-				dn = dnode_create(os, dn_block + idx, db,
-				    object, dnh);
-			}
+		/*
+		 * Allocated but otherwise free dnodes which would
+		 * be in the interior of a multi-slot dnodes need
+		 * to be freed.  Single slot dnodes can be safely
+		 * re-purposed as a performance optimization.
+		 */
+		if (slots > 1)
+			dnode_reclaim_slots(dnc, idx + 1, slots - 1);
+
+		dnh = &dnc->dnc_children[idx];
+		if (DN_SLOT_IS_PTR(dnh->dnh_dnode)) {
+			dn = dnh->dnh_dnode;
+		} else {
+			dn = dnode_create(os, dn_block + idx, db,
+			    object, dnh);
 		}
 
 		mutex_enter(&dn->dn_mtx);