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@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 9bc1019..05041f8 100644 (file)
--- 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;
                /*