----- On Dec 1, 2020, at 5:06 AM, Peter Zijlstra [email protected] 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/[email protected] > > 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 <[email protected]> >> --- >> 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
