linux-kernel - Re: block: flush queued bios when the process blocks

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Date:	Tue, 6 Oct 2015 10:26:48 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Mike Snitzer <snitzer@...hat.com>
cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	"Alasdair G. Kergon" <agk@...hat.com>,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: block: flush queued bios when the process blocks



On Tue, 6 Oct 2015, Mike Snitzer wrote:

> On Tue, Oct 06 2015 at  9:28am -0400,
> Mikulas Patocka <mpatocka@...hat.com> wrote:
> 
> > 
> > 
> > On Mon, 5 Oct 2015, Mike Snitzer wrote:
> > 
> > > Mikulas,
> > > 
> > > Could it be that cond_resched() wasn't unplugging?  As was
> > > recently raised in this thread: https://lkml.org/lkml/2015/9/18/378
> > > Chris Mason's patch from that thread fixed this	issue... I _think_ Linus
> > > has since committed Chris' work but I haven't kept my finger on the
> > > pulse of that issue.
> > 
> > I think it doesn't matter (regarding correctness) if cond_reched unplugs 
> > on not. If it didn't unplug, the process will be scheduled later, and it 
> > will eventually reach the point where it unplugs.
> 
> Couldn't the original deadlock you fixed (as described in your first
> patch) manifest when a new process is scheduled?

A process rescheduled with cond_reched is not stuck, it will be run later. 
It may contribute to increased request latency, but it can't contribute to 
a deadlock.

Mikulas
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