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

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Date:	Tue, 2 Oct 2007 11:03:14 -0700
From:	Zach Brown <zach.brown@...cle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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!


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)

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?

- z
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