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

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Message-ID: <20151006134753.GA30555@redhat.com>
Date:	Tue, 6 Oct 2015 09:47:53 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...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, 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?
 
> > FYI, I've put rebased versions of your 2 patches in my wip branch, see:
> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip
> > 
> > I tweaked the 2nd patch that adds bio_list to plug so that
> > generic_make_request's checks for in_generic_make_request isn't racey
> > (your original patch could happen to have current-plug set but
> > in_generic_make_request not yet set).
> 
> I don't recommend that second patch 
> (http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=5e740c2e45a767d8d6ef8ca36b0db705ef6259c4). 
> The patch just complicates things without adding any value. It's also not 
> correct because it plugs bios at places when bios aren't supposed to be 
> plugged

Avoiding another hook the the scheduler is a requirement (from Jens).
"without adding any value": it offers a different strategy for recording
bios to the bio_list by making it part of the plug.  The plug doesn't
actually block bios like requests are plugged.  What am I missing?
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