BRT: Relax brt_pending_apply() locking

Since brt_pending_apply() is running in syncing context, no other
brt_pending_tree accesses are possible for the TXG.  We don't need
to acquire brt_pending_lock here.

Reviewed-by: Pawel Jakub Dawidek <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #15955

module/zfs/brt.c

@@ -1473,26 +1473,23 @@ brt_pending_remove(spa_t *spa, const blkptr_t *bp, dmu_tx_t *tx)
 void
 brt_pending_apply(spa_t *spa, uint64_t txg)
 {
-	brt_t *brt;
+	brt_t *brt = spa->spa_brt;
 	brt_pending_entry_t *bpe;
 	avl_tree_t *pending_tree;
-	kmutex_t *pending_lock;
 	void *c;
 
 	ASSERT3U(txg, !=, 0);
 
-	brt = spa->spa_brt;
+	/*
+	 * We are in syncing context, so no other brt_pending_tree accesses
+	 * are possible for the TXG. Don't need to acquire brt_pending_lock.
+	 */
 	pending_tree = &brt->brt_pending_tree[txg & TXG_MASK];
-	pending_lock = &brt->brt_pending_lock[txg & TXG_MASK];
-
-	mutex_enter(pending_lock);
 
 	c = NULL;
 	while ((bpe = avl_destroy_nodes(pending_tree, &c)) != NULL) {
 		boolean_t added_to_ddt;
 
-		mutex_exit(pending_lock);
-
 		for (int i = 0; i < bpe->bpe_count; i++) {
 			/*
 			 * If the block has DEDUP bit set, it means that it
@@ -1510,10 +1507,7 @@ brt_pending_apply(spa_t *spa, uint64_t txg)
 		}
 
 		kmem_cache_free(brt_pending_entry_cache, bpe);
-		mutex_enter(pending_lock);
 	}
-
-	mutex_exit(pending_lock);
 }
 
 static void