Re: [PATCH 2/3] membarrier: Add an actual barrier before rseq_preempt()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Dec 1, 2020, at 5:06 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Mon, Nov 30, 2020 at 09:50:34AM -0800, Andy Lutomirski wrote:
>> It seems to be that most RSEQ membarrier users will expect any
>> stores done before the membarrier() syscall to be visible to the
>> target task(s).  While this is extremely likely to be true in
>> practice, nothing actually guarantees it by a strict reading of the
>> x86 manuals.  Rather than providing this guarantee by accident and
>> potentially causing a problem down the road, just add an explicit
>> barrier.
> 
> A very long time ago; when Jens introduced smp_call_function(), we had
> this discussion. At the time Linus said that receiving an interrupt had
> better be ordering, and if it is not, then it's up to the architecture
> to handle that before it gets into the common code.
> 
>  https://lkml.kernel.org/r/alpine.LFD.2.00.0902180744520.21686@localhost.localdomain
> 
> Maybe we want to revisit this now, but there might be a fair amount of
> code relying on all this by now.
> 
> Documenting it better might help.

Considering that we already have this in membarrier ipi_mb :

static void ipi_mb(void *info)
{
        smp_mb();       /* IPIs should be serializing but paranoid. */
}

I think it makes sense to add this same smp_mb() in the ipi_rseq if the expected
behavior is to order memory accesses as well, and have the same level of paranoia as
the ipi_mb.

Thanks,

Mathieu


> 
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>>  kernel/sched/membarrier.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>> 
>> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
>> index e23e74d52db5..7d98ef5d3bcd 100644
>> --- a/kernel/sched/membarrier.c
>> +++ b/kernel/sched/membarrier.c
>> @@ -40,6 +40,14 @@ static void ipi_mb(void *info)
>>  
>>  static void ipi_rseq(void *info)
>>  {
>> +	/*
>> +	 * Ensure that all stores done by the calling thread are visible
>> +	 * to the current task before the current task resumes.  We could
>> +	 * probably optimize this away on most architectures, but by the
>> +	 * time we've already sent an IPI, the cost of the extra smp_mb()
>> +	 * is negligible.
>> +	 */
>> +	smp_mb();
>>  	rseq_preempt(current);
>>  }
> 
> So I think this really isn't right.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com