linux-kernel - Re: [HACK] convert i_alloc_sem for direct

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Date:	Tue, 2 Oct 2007 14:12:03 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Zach Brown <zach.brown@...cle.com>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	mingo@...dmis.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [HACK] convert i_alloc_sem for direct_io.c craziness!


Hi Zach!

Thanks for the responses.

--
On Tue, 2 Oct 2007, Zach Brown wrote:
>
> On Oct 1, 2007, at 1:39 PM, Peter Zijlstra wrote:
>
> >
> > On Mon, 2007-10-01 at 12:52 -0700, Zach Brown wrote:
> >
> >> Do you have any suggestions for locking constructs that RT would
> >> prefer?
> >
> > Basically, anything that maps to a simple mutex. Anything more complex
> > gets real messy real quick.
>
> I'm worried that the aio+dio implementation of concurrent pending IOs
> just doesn't map well to PI.
>
> Would a hack with a mutex and counts help at all?  It seems like it
> would still have the same problem.  The count increments don't
> transfer ownership to the count decrements and the wake up.
>
> io submission from tasks:
> 	down(&inode->i_mutex);
> 	atomic_inc(&inode->in_flight);
> 	up(&inode->i_mutex);
>
> io completion from interrupts:
> 	if(atomic_dec_and_test(&inode->in_flight))
> 		wake_up(&inode->waiting);
>
> file allocation in tasks:
> 	down(&inode->i_mutex);
> 	wait_event(inode->waiting, atomic_read(&inode->in_flight) == 0);
> 	up(&inode->i_mutex);
>
> (yeah, yeah, starvation -- it's just a demonstration)

This looks more like a completion. Actually, completions don't have PI
either, but they are usually OK.

>
> In any case, this seems like it's not a very high priority now that
> RT has Steven's work-around.  If it does become a priority can you
> guys let linux-fsdevel know?

Actually, it may still be a high priority. We have one BUG currently that
seems to trigger starvation. But we don't know yet if it's a bug in the
test or in the FS code yet.  I don't know the full details yet, but we do
have someone currently looking at it. These tests are what found this
issue in the first place.

When more info becomes available, I will definitely CC the linux-fsdevel
list. Thanks for letting me know about it. (I usually get confused by all
the different lists that are out there).

-- Steve

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