blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing

[mirror_ubuntu-bionic-kernel.git] / block / blk-core.c

diff --git a/block/blk-core.c b/block/blk-core.c
index 991c1d6ef245f758260092033eac9267ebc39dbf..f2db628aa509a0e0a5935599ca715a96abf16c93 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -416,7 +416,8 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
  * In bypass mode, only the dispatch FIFO queue of @q is used.  This
  * function makes @q enter bypass mode and drains all requests which were
  * throttled or issued before.  On return, it's guaranteed that no request
- * is being throttled or has ELVPRIV set.
+ * is being throttled or has ELVPRIV set and blk_queue_bypass() %true
+ * inside queue or RCU read lock.
  */
 void blk_queue_bypass_start(struct request_queue *q)
 {
@@ -426,6 +427,8 @@ void blk_queue_bypass_start(struct request_queue *q)
        spin_unlock_irq(q->queue_lock);
 
        blk_drain_queue(q, false);
+       /* ensure blk_queue_bypass() is %true inside RCU read lock */
+       synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(blk_queue_bypass_start);
 
@@ -462,7 +465,15 @@ void blk_cleanup_queue(struct request_queue *q)
 
        spin_lock_irq(lock);
 
-       /* dead queue is permanently in bypass mode till released */
+       /*
+        * Dead queue is permanently in bypass mode till released.  Note
+        * that, unlike blk_queue_bypass_start(), we aren't performing
+        * synchronize_rcu() after entering bypass mode to avoid the delay
+        * as some drivers create and destroy a lot of queues while
+        * probing.  This is still safe because blk_release_queue() will be
+        * called only after the queue refcnt drops to zero and nothing,
+        * RCU or not, would be traversing the queue by then.
+        */
        q->bypass_depth++;
        queue_flag_set(QUEUE_FLAG_BYPASS, q);