linux-kernel - Re: boot stall regression due to blk-mq: use percpu

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Message-ID: <20140923060906.GA10547@lst.de>
Date:	Tue, 23 Sep 2014 08:09:06 +0200
From:	Christoph Hellwig <hch@....de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: boot stall regression due to blk-mq: use percpu_ref for mq
	usage count

On Tue, Sep 23, 2014 at 02:01:41AM -0400, Tejun Heo wrote:
> On Tue, Sep 23, 2014 at 07:59:24AM +0200, Christoph Hellwig wrote:
> > "[PATCHSET percpu/for-3.18] percpu_ref: implement switch_to_atomic/percpu()"
> > 
> > looks way to big for 3.17, and the regression was introduced in the 3.17
> > merge window.  I'm not sure what was broken before, but it defintively
> > survived a lot of testing.
> 
> Do we even care about fixing it for 3.17?  scsi-mq isn't enabled by
> default even for 3.18.  The open-coded percpu ref thing was subtly
> broken there.  It'd be difficult to trigger but I'm fairly sure it'd
> crap out in the wild once in a blue moon.

It's compiled in by default, and people are extremly eager to test it.
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