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Date:	Tue, 6 Oct 2015 09:47:53 -0400
From:	Mike Snitzer <>
To:	Mikulas Patocka <>
Cc:	Jens Axboe <>, Kent Overstreet <>,
	"Alasdair G. Kergon" <>,,
Subject: Re: block: flush queued bios when the process blocks

On Tue, Oct 06 2015 at  9:28am -0400,
Mikulas Patocka <> 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:
> > 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:
> >
> > 
> > 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 
> ( 
> 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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