linux-kernel - Re: [PATCH 3/3] ring-buffer: add design document

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Message-Id: <1244687581.8361.703.camel@yhuang-dev.sh.intel.com>
Date:	Thu, 11 Jun 2009 10:33:01 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Theodore Tso <tytso@....edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Christoph Hellwig <hch@...radead.org>,
	Li Zefan <lizf@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [PATCH 3/3] ring-buffer: add design document

On Thu, 2009-06-11 at 09:58 +0800, Steven Rostedt wrote:
> On Thu, 11 Jun 2009, Huang Ying wrote:
> 
> > On Thu, 2009-06-11 at 03:53 +0800, Steven Rostedt wrote:
> > > +
> > > +cmpxchg - hardware assisted atomic transaction that performs the following:
> > > +
> > > +   A = B iff previous A == C
> > > +
> > > +   R = cmpxchg(A, C, B) is saying that we replace A with B if and only if
> > > +      current A is equal to C, and we put the old (current) A into R
> > > +
> > > +   R gets the previous A regardless if A is updated with B or not.
> > > +
> > > +   To see if the update was successful a compare of R == C may be used.
> > 
> > As far as I know, some architectures have no hardware assisted (NMI
> > safe) cmpxchg. Is it OK to use cmpxchg in architecture-independent code?
> 
> I can fall back to the lock solution for those archs without cmpxchg. It 
> is NMI safe, because we do spin_trylock() in NMI context. If we fail to 
> acquire the lock in NMI context, we simply drop the packet.

Yes. For users do not care about packet drop, it is acceptable. But
please select the implementation at run-time instead of build time.
Because on some architecture such as ARM, whether CPU has cmpxchg
support is determined at run-time.

> Are these archs without cmpxchg and NMIs, a concern for you?

ARM has no cmpxchg until ARM v6, but it has NMI like mechanism named
FIQ.

Best Regards,
Huang Ying


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