blk-ioc: don't hold queue_lock for ioc_lookup_icq()

Currently issue io can grab queue_lock three times from bfq_bio_merge(),
bfq_limit_depth() and bfq_prepare_request(), the queue_lock is not
necessary if icq is already created because both queue and ioc can't be
freed before io issuing is done, hence remove the unnecessary queue_lock
and use rcu to protect radix tree lookup.

Noted this is also a prep patch to support request batch dispatching[1].

[1] https://lore.kernel.org/all/[email protected]/

Signed-off-by: Yu Kuai <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: YuKuai-huawei

block/bfq-iosched.c

@@ -454,17 +454,10 @@ static struct bfq_io_cq *icq_to_bic(struct io_cq *icq)
  */
 static struct bfq_io_cq *bfq_bic_lookup(struct request_queue *q)
 {
-	struct bfq_io_cq *icq;
-	unsigned long flags;
-
 	if (!current->io_context)
 		return NULL;
 
-	spin_lock_irqsave(&q->queue_lock, flags);
-	icq = icq_to_bic(ioc_lookup_icq(q));
-	spin_unlock_irqrestore(&q->queue_lock, flags);
-
-	return icq;
+	return icq_to_bic(ioc_lookup_icq(q));
 }
 
 /*
@@ -2457,15 +2450,8 @@ static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
 		unsigned int nr_segs)
 {
 	struct bfq_data *bfqd = q->elevator->elevator_data;
-	struct request *free = NULL;
-	/*
-	 * bfq_bic_lookup grabs the queue_lock: invoke it now and
-	 * store its return value for later use, to avoid nesting
-	 * queue_lock inside the bfqd->lock. We assume that the bic
-	 * returned by bfq_bic_lookup does not go away before
-	 * bfqd->lock is taken.
-	 */
 	struct bfq_io_cq *bic = bfq_bic_lookup(q);
+	struct request *free = NULL;
 	bool ret;
 
 	spin_lock_irq(&bfqd->lock);

block/blk-ioc.c

@@ -308,24 +308,23 @@ int __copy_io(unsigned long clone_flags, struct task_struct *tsk)
 
 #ifdef CONFIG_BLK_ICQ
 /**
- * ioc_lookup_icq - lookup io_cq from ioc
+ * ioc_lookup_icq - lookup io_cq from ioc in io issue path
  * @q: the associated request_queue
  *
  * Look up io_cq associated with @ioc - @q pair from @ioc.  Must be called
- * with @q->queue_lock held.
+ * from io issue path, either return NULL if current issue io to @q for the
+ * first time, or return a valid icq.
  */
 struct io_cq *ioc_lookup_icq(struct request_queue *q)
 {
 	struct io_context *ioc = current->io_context;
 	struct io_cq *icq;
 
-	lockdep_assert_held(&q->queue_lock);
-
 	/*
 	 * icq's are indexed from @ioc using radix tree and hint pointer,
-	 * both of which are protected with RCU.  All removals are done
-	 * holding both q and ioc locks, and we're holding q lock - if we
-	 * find a icq which points to us, it's guaranteed to be valid.
+	 * both of which are protected with RCU, io issue path ensures that
+	 * both request_queue and current task are valid, the found icq
+	 * is guaranteed to be valid until the io is done.
 	 */
 	rcu_read_lock();
 	icq = rcu_dereference(ioc->icq_hint);
@@ -419,10 +418,7 @@ struct io_cq *ioc_find_get_icq(struct request_queue *q)
 		task_unlock(current);
 	} else {
 		get_io_context(ioc);
-
-		spin_lock_irq(&q->queue_lock);
 		icq = ioc_lookup_icq(q);
-		spin_unlock_irq(&q->queue_lock);
 	}
 
 	if (!icq) {