linux-kernel - Re: scsi: non atomic allocation in mempool

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Message-ID: <54AAE81C.2080402@kernel.dk>
Date:	Mon, 05 Jan 2015 12:38:04 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@....de>
CC:	Sasha Levin <sasha.levin@...cle.com>, bvanassche@....org,
	hare@...e.de, JBottomley@...allels.com, linux-scsi@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: scsi: non atomic allocation in mempool_alloc in atomic context

On 01/05/2015 12:32 PM, Christoph Hellwig wrote:
> On Mon, Jan 05, 2015 at 12:00:58PM -0700, Jens Axboe wrote:
>> That's not quite true, the only guarantee is that it WILL execute on the
>> CPU (or CPUs) that are set in the mask. So unless it ends up offloading
>> the run to a specific workqueue, we'll disable preempt in the current
>> path before ->queue_rq() is called.
> 
> Oops.  Indeed, with those recent changes ->queue_rq can't safely block
> for memory allocatios anymore.
> 
> The patch below should fix it:
> 
> ---
> From: Christoph Hellwig <hch@....de>
> Subject: scsi: ->queue_rq can't sleep
> 
> Since Linux 3.19 blk-mq may disable preemption before calling into
> ->queue_rq, so we can't actually sleep anymore.

That was true in earlier kernels as well, going back a few versions at
least, preempt was disabled on calling __blk_mq_run_hw_queue(). Just
checked, and 3.16 and later have that as the behaviour. The only change
in 3.19 some shuffling around to avoid double preempt_disable in some
cases, it's now using get_cpu() and friends.

So we probably want do mark that as stable so we reach back to when
scsi-mq was added, unless the originally referenced patch getting rid of
the gfp_t mask didn't have the issue.


-- 
Jens Axboe

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