Re: [PATCH v2 2/4] membarrier: Add an actual barrier before rseq_preempt()

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

----- On Dec 2, 2020, at 10:35 AM, Andy Lutomirski luto@kernel.org wrote:

> It seems to me 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.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Andy Lutomirski <luto@kernel.org>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> ---
> kernel/sched/membarrier.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 5a40b3828ff2..6251d3d12abe 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -168,6 +168,14 @@ 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);
> }
> 
> --
> 2.28.0

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