linux-kernel - Re: [PATCH] sbitmap: Protect swap

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Date:   Mon, 14 Jan 2019 21:28:33 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, Clark Williams <williams@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sbitmap: Protect swap_lock from hardirq

On 1/14/19 8:59 PM, Ming Lei wrote:
> The original report is actually one real deadlock:
> 
>     [  106.132865]  Possible interrupt unsafe locking scenario:
>     [  106.132865]
>     [  106.133659]        CPU0                    CPU1
>     [  106.134194]        ----                    ----
>     [  106.134733]   lock(&(&sb->map[i].swap_lock)->rlock);
>     [  106.135318]                                local_irq_disable();
>     [  106.136014]                                lock(&sbq->ws[i].wait);
>     [  106.136747]                                lock(&(&hctx->dispatch_wait_lock)->rlock);
>     [  106.137742]   <Interrupt>
>     [  106.138110]     lock(&sbq->ws[i].wait);
> 
> Because we may call blk_mq_get_driver_tag() directly from
> blk_mq_dispatch_rq_list() without holding any lock, then HARDIRQ may come
> and the above DEADLOCK is triggered.
> 
> ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq") tries to fix
> this issue by using 'spin_lock_bh', which isn't enough because we complete
> request from hardirq context direclty in case of multiqueue.

Thanks Ming, I'll queue this up for shipping this week.

-- 
Jens Axboe