Re: [PATCH 3/3] membarrier: Propagate SYNC_CORE and RSEQ actions more carefully

[Peter Zijlstra]

On Tue, Dec 01, 2020 at 10:09:22AM -0800, Andy Lutomirski wrote:
> On Tue, Dec 1, 2020 at 2:16 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >  - membarrier() does not explicitly sync_core() remote CPUs either;
> > >    instead, it relies on the assumption that an IPI will result in a
> > >    core sync.  On x86, I think this may be true in practice, but
> > >    it's not architecturally reliable.  In particular, the SDM and
> > >    APM do not appear to guarantee that interrupt delivery is
> > >    serializing.
> >
> > Right, I don't think we rely on that, we do rely on interrupt delivery
> > providing order though -- as per the previous email.

order, not serializing.

> I looked for a bit, and I couldn't find anything in the SDM or APM to
> support this, and I would be rather surprised if other architectures
> synchronize their instruction streams on interrupt delivery.  On
> architectures without hardware I$ coherency and with actual fast
> interrupts, I would be surprised if interrupts ensured I$ coherency
> with prior writes from other cores.

Data, not I$. smp_mb() has nothing on I$. The claim is that smp_mb() at
the start of an IPI is pointless (as a means of ordering against the CPU
raising the IPI).

Doing smp_mb() before raising the IPI does make sense and is actually
done IIRC.