linux-kernel - [PATCH 2/2] blk-mq: use get_cpu/put_cpu instead of preempt_disable/preempt

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri,  7 Nov 2014 23:04:00 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Clark Williams <williams@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 2/2] blk-mq: use get_cpu/put_cpu instead of preempt_disable/preempt_enable

blk-mq is using preempt_disable/enable in order to ensure that the
queue runners are placed on the right CPU.  This does not work with
the RT patches, because __blk_mq_run_hw_queue takes a non-raw
spinlock with the preemption-disabled region.  If there is contention
on the lock, this violates the rules for preemption-disabled regions.

While this should be easily fixable within the RT patches just by doing
migrate_disable/enable, we can do better and document _why_ this
particular region runs with disabled preemption.  After the previous
patch, it is trivial to switch it to get/put_cpu; the RT patches then
can change it to get_cpu_light, which lets virtio-blk run under RT
kernels.

Cc: Jens Axboe <axboe@...nel.dk>
Cc: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Clark Williams <williams@...hat.com>
Tested-by: Clark Williams <williams@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 block/blk-mq.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c6192ba78950..a958851d9dd2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -829,14 +829,14 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 		return;

 	if (!async) {
-		preempt_disable();
-		if (cpumask_test_cpu(smp_processor_id(), hctx->cpumask)) {
+		int cpu = get_cpu();
+		if (cpumask_test_cpu(cpu, hctx->cpumask)) {
 			__blk_mq_run_hw_queue(hctx);
-			preempt_enable();
+			put_cpu();
 			return;
 		}

-		preempt_enable();
+		put_cpu();
 	}

 	if (hctx->queue->nr_hw_queues == 1)
-- 
2.1.0

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/