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

[Peter Zijlstra <peterz@infradead.org>]

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.

> 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.