linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b)

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Message-ID: <1263313659.28171.3810.camel@gandalf.stny.rr.com>
Date:	Tue, 12 Jan 2010 11:27:39 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	tglx@...utronix.de, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier (v3b)

On Tue, 2010-01-12 at 10:38 -0500, Mathieu Desnoyers wrote:

> > The UP-kernel case is handled by the #ifdef in sys_membarrier(), though
> > with a bit larger code footprint than the embedded guys would probably
> > prefer.  (Or is the compiler smart enough to omit these function given no
> > calls to them?  If not, recommend putting them under CONFIG_SMP #ifdef.)
> 
> Hrm, that's a bit odd. I agree that UP systems could simply return
> -ENOSYS for sys_membarrier, but then I wonder how userland could
> distinguish between:
> 
> - an old kernel not supporting sys_membarrier()
>   -> in this case we need to use the smp_mb() fallback on the read-side
>      and in synchronize_rcu().
> - a recent kernel supporting sys_membarrier(), CONFIG_SMP
>   -> can use the barrier() on read-side, call sys_membarrier upon
>      update.
> - a recent kernel supporting sys_membarrier, !CONFIG_SMP
>   -> calls to sys_membarrier() are not required, nor is barrier().
> 
> Or maybe we just postpone the userland smp_mb() question to another
> thread. This will eventually need to be addressed anyway. Maybe with a
> vgetmaxcpu() vsyscall.

I think Paul means to wrap all your other functions under the #ifdef.
What you have for sys_membarrier() is fine (just return 0 on UP) but you
also need to wrap the helper function above it under #ifdef CONFIG_SMP.
Don't rely on the compiler to optimize them out. If anything, you'll
probably get a bunch of warnings about static functions unused.

-- Steve


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