linux-kernel - Re: [PATCH 2/3] membarrier: Add an actual barrier before rseq

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Message-ID: <CALCETrXgKMamvjDZAjvZ36_N2BWRqniJRrtYz=89drMXc6A3LQ@mail.gmail.com>
Date:   Tue, 1 Dec 2020 09:55:17 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Anton Blanchard <anton@...abs.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/3] membarrier: Add an actual barrier before rseq_preempt()

On Tue, Dec 1, 2020 at 6:31 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> ----- On Dec 1, 2020, at 5:06 AM, Peter Zijlstra peterz@...radead.org 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/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.
>
> 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.

That was my reasoning.