linux-kernel - [PATCH RFC] blk-mq: Don't IPI requests on 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-next>] [day] [month] [year] [list]

Message-ID: <20201023110400.bx3uzsb7xy5jtsea@linutronix.de>
Date:   Fri, 23 Oct 2020 13:04:00 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     David Runge <dave@...epmap.de>
Cc:     linux-rt-users@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Wagner <dwagner@...e.de>
Subject: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT

blk_mq_complete_request_remote() will dispatch request completion to
another CPU via IPI if the CPU belongs to a different cache domain.

This breaks on PREEMPT_RT because the IPI function will complete the
request in IRQ context which includes acquiring spinlock_t typed locks.
Completing the IPI request in softirq on the remote CPU is probably less
efficient because it would require to wake ksoftirqd for this task
(which runs at SCHED_OTHER).

Ignoring the IPI request and completing the request locally is probably
the best option. It be completed either in the IRQ-thread or at the end
of the routine in softirq context.

Let blk_mq_complete_need_ipi() return that there is no need for IPI on
PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e37aa31332b70..99d2fb51e0e84 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -647,7 +647,7 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq)
 {
 	int cpu = raw_smp_processor_id();

-	if (!IS_ENABLED(CONFIG_SMP) ||
+	if (!IS_ENABLED(CONFIG_SMP) || IS_ENABLED(CONFIG_PREEMPT_RT) ||
 	    !test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
 		return false;

-- 
2.28.0