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SubjectRe: 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)
+ if (unlikely(current->io_context && current->io_context->plugged))
+ blk_replug_current_nested();
profile_hit(SCHED_PROFILING, __builtin_return_address(0));

Jens Axboe

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