linux-kernel - Re: 2.6.20-rc6-mm3

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Date:	Mon, 5 Feb 2007 13:56:43 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...l.org>, David Chinner <dgc@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.20-rc6-mm3

On Mon, Feb 05 2007, Jens Axboe wrote:
> On Mon, Feb 05 2007, Jens Axboe wrote:
> > On Thu, Feb 01 2007, Christoph Lameter wrote:
> > > On Thu, 1 Feb 2007, Jens Axboe wrote:
> > > 
> > > > That looks like barriers, could you try with those disabled? Sorry for
> > > > making you go through this, I can't debug and fix it myself before
> > > > monday.
> > > 
> > > Disabling barriers + your patch works. Modified /etc/fstab and added a 
> > > nobarrier option to the root filesystem. If I take your patch out then the 
> > > systems hangs again.
> > 
> > I can't reproduce this. Can you see if this debug patch catches
> > anything? You need to enable barriers again.
> 
> Nevermind, that was too aggressive. I'll come up with a better debug
> patch.

Alright, try this one. It should show whether this is a missing unplug
or not (which I think it is, hence the stall in qrcu sync). A process
may legitimately block with plugged requests, that sometimes happens for
bio/rq allocation etc. In that case we do want to unplug anyway though,
as I don't think we should hold requests plugged even for a merge if we
are going to block. And the below means we can move this out of
io_schedule() and eliminate the io_schedule() requirement that I'm not
too fond of, as I don't want to reintroduce all the problems we had with
missing unplugs in the 2.4 kernels.

But for now this is just a debug test, can you see if xfs with barriers
for that kernel now works as expected?

diff --git a/kernel/sched.c b/kernel/sched.c
index e209901..6a54e4d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3434,6 +3434,8 @@ asmlinkage void __sched schedule(void)
 			print_irqtrace_events(current);
 		dump_stack();
 	}
+	if (unlikely(current->io_context && current->io_context->plugged))
+		blk_replug_current_nested();
 	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
 
 need_resched:

-- 
Jens Axboe

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