linux-kernel - [PATCH BUGFIX IMPROVEMENT V2 2/7] block, bfq: fix rq_in_driver check in bfq_update_inject

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Date:   Tue, 25 Jun 2019 07:12:44 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        bottura.nicola95@...il.com, srivatsa@...il.mit.edu,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX IMPROVEMENT V2 2/7] block, bfq: fix rq_in_driver check in bfq_update_inject_limit

One of the cases where the parameters for injection may be updated is
when there are no more in-flight I/O requests. The number of in-flight
requests is stored in the field bfqd->rq_in_driver of the descriptor
bfqd of the device. So, the controlled condition is
bfqd->rq_in_driver == 0.

Unfortunately, this is wrong because, the instruction that checks this
condition is in the code path that handles the completion of a
request, and, in particular, the instruction is executed before
bfqd->rq_in_driver is decremented in such a code path.

This commit fixes this issue by just replacing 0 with 1 in the
comparison.

Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@...il.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@...il.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 9bc10198ddff..05041f84b8da 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5481,8 +5481,14 @@ static void bfq_update_inject_limit(struct bfq_data *bfqd,
 	 * total service time, and there seem to be the right
 	 * conditions to do it, or we can lower the last base value
 	 * computed.
+	 *
+	 * NOTE: (bfqd->rq_in_driver == 1) means that there is no I/O
+	 * request in flight, because this function is in the code
+	 * path that handles the completion of a request of bfqq, and,
+	 * in particular, this function is executed before
+	 * bfqd->rq_in_driver is decremented in such a code path.
 	 */
-	if ((bfqq->last_serv_time_ns == 0 && bfqd->rq_in_driver == 0) ||
+	if ((bfqq->last_serv_time_ns == 0 && bfqd->rq_in_driver == 1) ||
 	    tot_time_ns < bfqq->last_serv_time_ns) {
 		bfqq->last_serv_time_ns = tot_time_ns;
 		/*
-- 
2.20.1