Add a "try" operation for range locks

zfs_rangelock_tryenter() bails immediately instead of waiting for the
lock to become available.  This will be used to resolve a deadlock in
the FreeBSD page-in code.  No functional change intended.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Mark Johnston <[email protected]>
Closes #10519

1 files changed, 47 insertions, 18 deletions

diff --git a/module/zfs/zfs_rlock.c b/module/zfs/zfs_rlock.c
index 091b37f9f..06a5e031a 100644
--- a/module/zfs/zfs_rlock.c
+++ b/module/zfs/zfs_rlock.c
@@ -150,10 +150,12 @@ zfs_rangelock_fini(zfs_rangelock_t *rl)
 }
 
 /*
- * Check if a write lock can be grabbed, or wait and recheck until available.
+ * Check if a write lock can be grabbed.  If not, fail immediately or sleep and
+ * recheck until available, depending on the value of the "nonblock" parameter.
  */
-static void
-zfs_rangelock_enter_writer(zfs_rangelock_t *rl, zfs_locked_range_t *new)
+static boolean_t
+zfs_rangelock_enter_writer(zfs_rangelock_t *rl, zfs_locked_range_t *new,
+    boolean_t nonblock)
 {
 	avl_tree_t *tree = &rl->rl_tree;
 	zfs_locked_range_t *lr;
@@ -183,7 +185,7 @@ zfs_rangelock_enter_writer(zfs_rangelock_t *rl, zfs_locked_range_t *new)
 		 */
 		if (avl_numnodes(tree) == 0) {
 			avl_add(tree, new);
-			return;
+			return (B_TRUE);
 		}
 
 		/*
@@ -204,8 +206,10 @@ zfs_rangelock_enter_writer(zfs_rangelock_t *rl, zfs_locked_range_t *new)
 			goto wait;
 
 		avl_insert(tree, new, where);
-		return;
+		return (B_TRUE);
 wait:
+		if (nonblock)
+			return (B_FALSE);
 		if (!lr->lr_write_wanted) {
 			cv_init(&lr->lr_write_cv, NULL, CV_DEFAULT, NULL);
 			lr->lr_write_wanted = B_TRUE;
@@ -391,10 +395,12 @@ zfs_rangelock_add_reader(avl_tree_t *tree, zfs_locked_range_t *new,
 }
 
 /*
- * Check if a reader lock can be grabbed, or wait and recheck until available.
+ * Check if a reader lock can be grabbed.  If not, fail immediately or sleep and
+ * recheck until available, depending on the value of the "nonblock" parameter.
  */
-static void
-zfs_rangelock_enter_reader(zfs_rangelock_t *rl, zfs_locked_range_t *new)
+static boolean_t
+zfs_rangelock_enter_reader(zfs_rangelock_t *rl, zfs_locked_range_t *new,
+    boolean_t nonblock)
 {
 	avl_tree_t *tree = &rl->rl_tree;
 	zfs_locked_range_t *prev, *next;
@@ -415,6 +421,8 @@ retry:
 	 */
 	if (prev && (off < prev->lr_offset + prev->lr_length)) {
 		if ((prev->lr_type == RL_WRITER) || (prev->lr_write_wanted)) {
+			if (nonblock)
+				return (B_FALSE);
 			if (!prev->lr_read_wanted) {
 				cv_init(&prev->lr_read_cv,
 				    NULL, CV_DEFAULT, NULL);
@@ -439,6 +447,8 @@ retry:
 		if (off + len <= next->lr_offset)
 			goto got_lock;
 		if ((next->lr_type == RL_WRITER) || (next->lr_write_wanted)) {
+			if (nonblock)
+				return (B_FALSE);
 			if (!next->lr_read_wanted) {
 				cv_init(&next->lr_read_cv,
 				    NULL, CV_DEFAULT, NULL);
@@ -457,6 +467,7 @@ got_lock:
 	 * locks and bumping ref counts (r_count).
 	 */
 	zfs_rangelock_add_reader(tree, new, prev, where);
+	return (B_TRUE);
 }
 
 /*
@@ -464,11 +475,12 @@ got_lock:
  * (RL_WRITER or RL_APPEND).  If RL_APPEND is specified, rl_cb() will convert
  * it to a RL_WRITER lock (with the offset at the end of the file).  Returns
  * the range lock structure for later unlocking (or reduce range if the
- * entire file is locked as RL_WRITER).
+ * entire file is locked as RL_WRITER), or NULL if nonblock is true and the
+ * lock could not be acquired immediately.
  */
-zfs_locked_range_t *
-zfs_rangelock_enter(zfs_rangelock_t *rl, uint64_t off, uint64_t len,
-    zfs_rangelock_type_t type)
+static zfs_locked_range_t *
+zfs_rangelock_enter_impl(zfs_rangelock_t *rl, uint64_t off, uint64_t len,
+    zfs_rangelock_type_t type, boolean_t nonblock)
 {
 	zfs_locked_range_t *new;
 
@@ -491,18 +503,34 @@ zfs_rangelock_enter(zfs_rangelock_t *rl, uint64_t off, uint64_t len,
 		/*
 		 * First check for the usual case of no locks
 		 */
-		if (avl_numnodes(&rl->rl_tree) == 0)
+		if (avl_numnodes(&rl->rl_tree) == 0) {
 			avl_add(&rl->rl_tree, new);
-		else
-			zfs_rangelock_enter_reader(rl, new);
-	} else {
-		/* RL_WRITER or RL_APPEND */
-		zfs_rangelock_enter_writer(rl, new);
+		} else if (!zfs_rangelock_enter_reader(rl, new, nonblock)) {
+			kmem_free(new, sizeof (*new));
+			new = NULL;
+		}
+	} else if (!zfs_rangelock_enter_writer(rl, new, nonblock)) {
+		kmem_free(new, sizeof (*new));
+		new = NULL;
 	}
 	mutex_exit(&rl->rl_lock);
 	return (new);
 }
 
+zfs_locked_range_t *
+zfs_rangelock_enter(zfs_rangelock_t *rl, uint64_t off, uint64_t len,
+    zfs_rangelock_type_t type)
+{
+	return (zfs_rangelock_enter_impl(rl, off, len, type, B_FALSE));
+}
+
+zfs_locked_range_t *
+zfs_rangelock_tryenter(zfs_rangelock_t *rl, uint64_t off, uint64_t len,
+    zfs_rangelock_type_t type)
+{
+	return (zfs_rangelock_enter_impl(rl, off, len, type, B_TRUE));
+}
+
 /*
  * Safely free the zfs_locked_range_t.
  */
@@ -657,6 +685,7 @@ zfs_rangelock_reduce(zfs_locked_range_t *lr, uint64_t off, uint64_t len)
 EXPORT_SYMBOL(zfs_rangelock_init);
 EXPORT_SYMBOL(zfs_rangelock_fini);
 EXPORT_SYMBOL(zfs_rangelock_enter);
+EXPORT_SYMBOL(zfs_rangelock_tryenter);
 EXPORT_SYMBOL(zfs_rangelock_exit);
 EXPORT_SYMBOL(zfs_rangelock_reduce);
 #endif