linux-kernel - Re: [PATCH] mmc: queue: prevent soft lockups on PREEMPT=n

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Date:	Tue, 16 Jun 2015 10:06:40 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Rabin Vincent <rabin.vincent@...s.com>
Cc:	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabinv@...s.com>
Subject: Re: [PATCH] mmc: queue: prevent soft lockups on PREEMPT=n

On 14 June 2015 at 19:26, Rabin Vincent <rabin.vincent@...s.com> wrote:
> On systems with CONFIG_PREEMPT=n, under certain circumstances, mmcqd
> can continuously process requests for several seconds without blocking,
> triggering the soft lockup watchdog.  For example, this can happen if
> mmcqd runs on the CPU which services the controller's interrupt, and
> a process on a different CPU continuously writes to the MMC block
> device.
>
>  NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [mmcqd/0:664]
>  CPU: 0 PID: 664 Comm: mmcqd/0 Not tainted 4.1.0-rc7+ #4
>  PC is at _raw_spin_unlock_irqrestore+0x24/0x28
>  LR is at mmc_start_request+0x104/0x134
>  ...
>  [<805112a8>] (_raw_spin_unlock_irqrestore) from [<803db664>] (mmc_start_request+0x104/0x134)
>  [<803db664>] (mmc_start_request) from [<803dc008>] (mmc_start_req+0x274/0x394)
>  [<803dc008>] (mmc_start_req) from [<803eb2c4>] (mmc_blk_issue_rw_rq+0xd0/0xb98)
>  [<803eb2c4>] (mmc_blk_issue_rw_rq) from [<803ebe8c>] (mmc_blk_issue_rq+0x100/0x470)
>  [<803ebe8c>] (mmc_blk_issue_rq) from [<803ecab8>] (mmc_queue_thread+0xd0/0x170)
>  [<803ecab8>] (mmc_queue_thread) from [<8003fd14>] (kthread+0xe0/0xfc)
>  [<8003fd14>] (kthread) from [<8000f768>] (ret_from_fork+0x14/0x2c)
>
> Fix it by adding a cond_resched() in the request handling loop so that
> other processes get a chance to run.
>
> Signed-off-by: Rabin Vincent <rabin.vincent@...s.com>

Thanks, applied!

Kind regards
Uffe

> ---
>  drivers/mmc/card/queue.c |    1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/mmc/card/queue.c b/drivers/mmc/card/queue.c
> index 8efa368..e78ae97 100644
> --- a/drivers/mmc/card/queue.c
> +++ b/drivers/mmc/card/queue.c
> @@ -69,6 +69,7 @@ static int mmc_queue_thread(void *d)
>                         set_current_state(TASK_RUNNING);
>                         cmd_flags = req ? req->cmd_flags : 0;
>                         mq->issue_fn(mq, req);
> +                       cond_resched();
>                         if (mq->flags & MMC_QUEUE_NEW_REQUEST) {
>                                 mq->flags &= ~MMC_QUEUE_NEW_REQUEST;
>                                 continue; /* fetch again */
> --
> 1.7.10.4
>
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