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

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Message-ID: <alpine.DEB.1.10.0808291256310.17917@gandalf.stny.rr.com>
Date:	Fri, 29 Aug 2008 12:58:05 -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:
> 
> Yeah, ideas crossed in the mail ;)
> 
> 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 ;)
> 
> 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,
> 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 answered this on IRC, but this is for the rest of those reading this 
thread.

Userspace (vsyscalls) can only use raw_seqlock_t. And only the read 
version for that matter. Since the read of raw_seqlock_t is just that, a 
read, no writes, and no jumping to other functions on contention.

The vsyscalls should never use the -rt seqlock_t. Not modifying the raws 
here should make us golden.

-- Steve

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