linux-kernel - Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9)

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Message-ID: <20100316131336.GB24808@elte.hu>
Date:	Tue, 16 Mar 2010 14:13:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	Chris Friesen <cfriesen@...tel.com>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory
 barrier (v9)


* Nick Piggin <npiggin@...e.de> wrote:

> On Tue, Mar 16, 2010 at 08:36:35AM +0100, Ingo Molnar wrote:
> > 
> > * Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > 
> > > Unless this question is answered, Ingo's SA_RUNNING signal proposal, as 
> > > appealing as it may look at a first glance, falls into the 
> > > "fundamentally broken" category. [...]
> > 
> > How is it different from your syscall? I.e. which lines of code make the 
> > difference? We could certainly apply the (trivial) barrier change to 
> > context_switch().
> 
> I think it is just easy for userspace to misuse or think it does something 
> that it doesn't (because of races).

That wasnt my question though. The question i asked Mathieu was to show how 
SA_RUNNING is "fundamentally broken" for librcu use while sys_membarrier() is 
not?

This is really what he claims above. (i preserved the quote)

It must be a misunderstanding either on my side or on his side. (Once that is 
cleared we can discuss further usecases for SA_RUNNING.)

Thanks,

	Ingo
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