dm bufio: use the acquire memory barrier when testing for B_READING

The function test_bit doesn't provide any memory barrier. It may be
possible that the read requests that follow test_bit(B_READING, &b->state)
are reordered before the test, reading invalid data that existed before
B_READING was cleared.

Fix this bug by changing test_bit to test_bit_acquire. This is
particularly important on arches with weak(er) memory ordering
(e.g. arm64).

Depends-On: 8238b457 ("wait_on_bit: add an acquire memory barrier")
Depends-On: d6ffe606 ("provide arch_test_bit_acquire for architectures that define test_bit")
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-bufio.c

@@ -795,7 +795,8 @@ static void __make_buffer_clean(struct dm_buffer *b)
 {
 	BUG_ON(b->hold_count);
 
-	if (!b->state)	/* fast case */
+	/* smp_load_acquire() pairs with read_endio()'s smp_mb__before_atomic() */
+	if (!smp_load_acquire(&b->state))	/* fast case */
 		return;
 
 	wait_on_bit_io(&b->state, B_READING, TASK_UNINTERRUPTIBLE);
@@ -816,7 +817,7 @@ static struct dm_buffer *__get_unclaimed_buffer(struct dm_bufio_client *c)
 		BUG_ON(test_bit(B_DIRTY, &b->state));
 
 		if (static_branch_unlikely(&no_sleep_enabled) && c->no_sleep &&
-		    unlikely(test_bit(B_READING, &b->state)))
+		    unlikely(test_bit_acquire(B_READING, &b->state)))
 			continue;
 
 		if (!b->hold_count) {
@@ -1058,7 +1059,7 @@ static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
 	 * If the user called both dm_bufio_prefetch and dm_bufio_get on
 	 * the same buffer, it would deadlock if we waited.
 	 */
-	if (nf == NF_GET && unlikely(test_bit(B_READING, &b->state)))
+	if (nf == NF_GET && unlikely(test_bit_acquire(B_READING, &b->state)))
 		return NULL;
 
 	b->hold_count++;
@@ -1218,7 +1219,7 @@ void dm_bufio_release(struct dm_buffer *b)
 		 * invalid buffer.
 		 */
 		if ((b->read_error || b->write_error) &&
-		    !test_bit(B_READING, &b->state) &&
+		    !test_bit_acquire(B_READING, &b->state) &&
 		    !test_bit(B_WRITING, &b->state) &&
 		    !test_bit(B_DIRTY, &b->state)) {
 			__unlink_buffer(b);
@@ -1479,7 +1480,7 @@ EXPORT_SYMBOL_GPL(dm_bufio_release_move);
 
 static void forget_buffer_locked(struct dm_buffer *b)
 {
-	if (likely(!b->hold_count) && likely(!b->state)) {
+	if (likely(!b->hold_count) && likely(!smp_load_acquire(&b->state))) {
 		__unlink_buffer(b);
 		__free_buffer_wake(b);
 	}
@@ -1639,7 +1640,7 @@ static bool __try_evict_buffer(struct dm_buffer *b, gfp_t gfp)
 {
 	if (!(gfp & __GFP_FS) ||
 	    (static_branch_unlikely(&no_sleep_enabled) && b->c->no_sleep)) {
-		if (test_bit(B_READING, &b->state) ||
+		if (test_bit_acquire(B_READING, &b->state) ||
 		    test_bit(B_WRITING, &b->state) ||
 		    test_bit(B_DIRTY, &b->state))
 			return false;