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Date: Tue, 17 Mar 2015 21:06:35 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Nicholas Miell <nmiell@comcast.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Howells <dhowells@redhat.com>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory
 barrier (x86) (v12)
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On Tue, 17 Mar 2015 12:46:41 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:


> > Would that help?
> > 
> > 	this_cpu_write(saved_next, next);
> > 	rq = context_switch(rq, prev, next);
> > 	rq->curr = this_cpu_read(saved_next);
> 
> Assuming there is a full memory barrier (e.g. load_cr3) within
> context_switch, it would help for ordering memory accesses that
> are performed prior to the preemption, but not for memory accesses
> to be performed immediately after return to userspace from preemption.

Hmm, I was thinking that there was a spin_unlock(rq->lock) after that,
but it appears that context_switch() does the unlock. If there was an
spin_unlock() after this code, then it could work. There's always
setting the rq->curr in the context_switch() call itself.

-- Steve