linux-kernel - Re: [PATCH] i386 Time: Avoid PIT SMP lockups

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Date:	Wed, 11 Oct 2006 16:03:28 -0700
From:	Andrew Morton <akpm@...l.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i386 Time: Avoid PIT SMP lockups

On Wed, 11 Oct 2006 15:48:31 -0700
john stultz <johnstul@...ibm.com> wrote:

> > Wouldn't it be better to fix the livelock?  What's causing it?
> 
> I spent a few days trying to narrow this down, and I haven't been able
> to do so to my satisfaction.
> 
> At this point, my suspicion is that because the PIT io-read is very slow
> (~18us), and done while holding a lock. It would be possible that one
> cpu calling gettimeofday would do the following:
> 
> grab xtime sequence read lock
> grab i8253 spin lock
> do port io (very slow)
> release i8253 spin lock
> realize xtime has been grabed and repeat
> 
> While another cpu does the following after in a timer interrupt:
> Grabs xtime sequence write lock
> spins trying to grab i8253 spin lock
> 
> Assuming the first thread can reacquire the i8253 lock before the
> second, you could have both threads potentially spinning forever.

Is there any actual need to hold xtime_lock while doing the port IO?  I'd
have thought it would suffice to do

	temp = port_io
	write_seqlock(xtime_lock);
	xtime = muck_with(temp);
	write_sequnlock(xtime_lock);

?
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