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[BUGFIX,2/3] block, bfq: remove wrong check in bfq_requests_merged

Message ID 20180531132313.2986-3-paolo.valente@linaro.org
State Accepted
Commit 8abfa4d6fd1537a570d99c52e9a683b759c27a45
Headers show
Series three fixes for the function bfq_requests_merged | expand

Commit Message

Paolo Valente May 31, 2018, 1:23 p.m. UTC 
The request rq passed to the function bfq_requests_merged is always in
a bfq_queue, so the check !RB_EMPTY_NODE(&rq->rb_node) at the
beginning of bfq_requests_merged always succeeds, and the control
flow systematically skips to the end of the function.  This implies
that the body of the function is never executed, i.e., the
repositioning of rq is never performed.

On the opposite end, a control is missing in the body of the function:
'next' must be removed only if it is inside a bfq_queue.

This commit removes the wrong check on rq, and adds the missing check
on 'next'. In addition, this commit adds comments on
bfq_requests_merged.

Signed-off-by: Filippo Muzzini <filippo.muzzini@outlook.it>

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>

---
 block/bfq-iosched.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

-- 
2.16.1
diff mbox series

Patch

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 1f0951d36424..df2a9633cf4a 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -1891,14 +1891,27 @@  static void bfq_request_merged(struct request_queue *q, struct request *req,
 	}
 }
 
+/*
+ * This function is called to notify the scheduler that the requests
+ * rq and 'next' have been merged, with 'next' going away.  BFQ
+ * exploits this hook to address the following issue: if 'next' has a
+ * fifo_time lower that rq, then the fifo_time of rq must be set to
+ * the value of 'next', to not forget the greater age of 'next'.
+ * Moreover 'next' may be in a bfq_queue, in this case it must be
+ * removed.
+ *
+ * NOTE: in this function we assume that rq is in a bfq_queue, basing
+ * on that rq is picked from the hash table q->elevator->hash, which,
+ * in its turn, is filled only with I/O requests present in
+ * bfq_queues, while BFQ is in use for the request queue q. In fact,
+ * the function that fills this hash table (elv_rqhash_add) is called
+ * only by bfq_insert_request.
+ */
 static void bfq_requests_merged(struct request_queue *q, struct request *rq,
 				struct request *next)
 {
 	struct bfq_queue *bfqq = RQ_BFQQ(rq), *next_bfqq = RQ_BFQQ(next);
 
-	if (!RB_EMPTY_NODE(&rq->rb_node))
-		goto end;
-
 	/*
 	 * If next and rq belong to the same bfq_queue and next is older
 	 * than rq, then reposition rq in the fifo (by substituting next
@@ -1919,10 +1932,11 @@  static void bfq_requests_merged(struct request_queue *q, struct request *rq,
 	if (bfqq->next_rq == next)
 		bfqq->next_rq = rq;
 
-	bfq_remove_request(q, next);
-	bfqg_stats_update_io_remove(bfqq_group(bfqq), next->cmd_flags);
+	if (!RB_EMPTY_NODE(&next->rb_node)) {
+		bfq_remove_request(q, next);
+		bfqg_stats_update_io_remove(bfqq_group(bfqq), next->cmd_flags);
+	}
 
-end:
 	bfqg_stats_update_io_merged(bfqq_group(bfqq), next->cmd_flags);
 }