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

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Date:	Fri, 29 Aug 2008 12:53:13 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Gregory Haskins <gregory.haskins@...il.com>,
	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


The subject forgot to add "RT" in the brackets.

On Fri, 29 Aug 2008, Andi Kleen wrote:

> > I could just force all of the seqbegins to hit the slowpath by hacking
> > the code and see what happens (aside from slowing down, of course ;)
> 
> Only if you don't believe it will really crash? I think it's pretty
> clear even without trying it.
> 
> > Question: Which seqlock_t does userspace use?  I assume it uses
> > seqlock_t and not raw_seqlock_t. 
> 
> > But the only reason that I ask is that
> > I converted raw_seqlock_t to use the new style as well to be consistent,
> 
> There's no raw_seqlock_t anywhere in mainline?

Nope, raw_seqlock_t in -rt is equivelant to seqlock_t in mainline.

> 
> Anyways the variable is declared (in mainline) in asm-x86/vgtod.h 
> 
> > even though it is not strictly necessary for the same reasons.  So if
> > perchance userspace uses the raw variant, I could solve this issue by
> > only re-working the seqlock_t variant.  Kind of a long shot, but figured
> > I would mention it :)
> 
> I guess you could define a new seqlock_t which is explicitely user space
> safe. That might avoid such issues in the future. But then
> that would likely require some code duplication and be ugly.
> 
> On the other hand whatever problem you fixing in the kernel
> (to be honest it's still unclear to me what the problem is)
> needs to be likely fixed for the userland lock too.

I'm not convinced that the raw_seqlocks (mainline normal seqlocks) has a 
problem anyway.

-- Steve

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