dm bufio: fix some cases where the code sleeps with spinlock held

Commit b32d45824aa7 ("dm bufio: Add DM_BUFIO_CLIENT_NO_SLEEP flag")
added a "NO_SLEEP" mode, it replaces a mutex with a spinlock, and it
is only usable when the device is in read-only mode (because the write
path may be sleeping while holding the dm_bufio_client lock).

However, there are still two points where the code could sleep even in
read-only mode. One is in __get_unclaimed_buffer -> __make_buffer_clean.
The other is in __try_evict_buffer -> __make_buffer_clean. These functions
will call __make_buffer_clean which sleeps if the buffer is being read.

Fix these cases so that if c->no_sleep is set __make_buffer_clean
will not be called and the buffer will be skipped instead.

Fixes: b32d45824aa7 ("dm bufio: Add DM_BUFIO_CLIENT_NO_SLEEP flag")
Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-bufio.c

···
 		BUG_ON(test_bit(B_WRITING, &b->state));
 		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)))
+			continue;
+
 		if (!b->hold_count) {
 			__make_buffer_clean(b);
 			__unlink_buffer(b);
···
 		}
 		cond_resched();
 	}
+
+	if (static_branch_unlikely(&no_sleep_enabled) && c->no_sleep)
+		return NULL;
 
 	list_for_each_entry_reverse(b, &c->lru[LIST_DIRTY], lru_list) {
 		BUG_ON(test_bit(B_READING, &b->state));
···
  */
 static bool __try_evict_buffer(struct dm_buffer *b, gfp_t gfp)
 {
-	if (!(gfp & __GFP_FS)) {
+	if (!(gfp & __GFP_FS) ||
+	    (static_branch_unlikely(&no_sleep_enabled) && b->c->no_sleep)) {
 		if (test_bit(B_READING, &b->state) ||
 		    test_bit(B_WRITING, &b->state) ||
 		    test_bit(B_DIRTY, &b->state))