Re: boot stall regression due to blk-mq: use percpu_ref for mq usage count

From: Tejun Heo
Date: Tue Sep 23 2014 - 02:12:02 EST

On Tue, Sep 23, 2014 at 08:09:06AM +0200, Christoph Hellwig wrote:
> 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.

Ugh, I don't know. It's not like we have a very good baseline we can
go back to and reverting it for -stable and then redoing it seems
kinda excessive for a yet experimental feature. Jens?

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