Messages in this thread | | | Date | Wed, 3 Mar 2010 16:36:14 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH -tip 2/2] x86: Issue at least one memory barrier in stop_machine_text_poke(). |
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* 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
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