Re: [PATCH -tip 2/2] x86: Issue at least one memory barrier in stop_machine_text_poke().

[Masami Hiramatsu <mhiramat@redhat.com>]

Mathieu Desnoyers wrote:
> * 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.

Ah, OK. Indeed, volatile is for compiler...

Thank you,

-- 
Masami Hiramatsu
e-mail: mhiramat@redhat.com