linux-kernel - Re: [PATCH] seqlock: serialize against writers

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Date:	Fri, 29 Aug 2008 12:34:24 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Gregory Haskins <gregory.haskins@...il.com>
cc:	Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] seqlock: serialize against writers



On Fri, 29 Aug 2008, Gregory Haskins wrote:

> Andi Kleen wrote:
> >> Im running it on a x86_64 box as we speak.  How can I tell if there is a
> >> certain mode that is permitting this?
> >>     
> >
> > If the boot up says you're running with PMtimer then it uses the fallback
> > (usually happens on pre Fam10h AMD boxes). A typical Intel box
> > would use the faster ring 3 only TSC path and then explode with your
> > change I bet. 
> >
> > Or step with gdb through gettimeofday() and see if it does a syscall.
> >
> > -Andi
> >   
> 
> It seems to be running fine with no indication it has fallen back. 
> Perhaps I need a certain workload to bring out the issue?

Perhaps you never hit the slow path in userland. That's the only place it 
would write. Perhaps add a dummy static variable in the fast path, and 
write to it. See if that crashes you apps.

-- Steve
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