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

[Andy Lutomirski <luto@kernel.org>]

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.

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);
 }
 
-- 
2.28.0