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Re: [PATCH -tip 2/2] x86: Issue at least one memory barrier in stop_machine_text_poke().
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Masami Hiramatsu <mhiramat at redhat dot com>
- Cc: Ingo Molnar <mingo at elte dot hu>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, Jason Baron <jbaron at redhat dot com>
- Date: Wed, 3 Mar 2010 16:36:14 -0500
- Subject: Re: [PATCH -tip 2/2] x86: Issue at least one memory barrier in stop_machine_text_poke().
- References: <20100303212656.25645.48466.stgit@localhost6.localdomain6> <20100303212703.25645.50986.stgit@localhost6.localdomain6>
* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Fix stop_machine_text_poke() to issue smp_mb() before exiting waiting
> loop. Also, use ACCESS_ONCE() to check a flag according to Mathieu's
> comment.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Jason Baron <jbaron@redhat.com>
> ---
>
> arch/x86/kernel/alternative.c | 6 +++---
> 1 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 635e4f4..3236a11 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -596,9 +596,9 @@ static int __kprobes stop_machine_text_poke(void *data)
> smp_wmb(); /* Make sure other cpus see that this has run */
> wrote_text = 1;
> } else {
> - while (!wrote_text)
> - smp_rmb();
> - sync_core();
> + while (!ACCESS_ONCE(wrote_text))
Well, cpu_relax() has a "memory" clobber, so the access once is not required to
ensure the variable is re-read. And, sorry to contradict my previous statement
somewhat, but given that we don't care if the compiler fetches wrote_text in
chunks or not, ACCESS_ONCE() is not required here. So rather than leaving people
wondering why we put an ACCESS_ONCE() here, it's probably better to leave it
out.
Thanks,
Mathieu
> + cpu_relax();
> + smp_mb(); /* Load wrote_text before following execution */
> }
>
> flush_icache_range((unsigned long)tpp->addr,
>
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
>
> e-mail: mhiramat@redhat.com
--
Mathieu Desnoyers
Operating System Efficiency Consultant
EfficiOS Inc.
http://www.efficios.com