linux-kernel - Re: [PATCH v3 2/4] membarrier: Add an actual barrier before rseq

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Message-ID: <CALCETrWPLcNk5XsartvjPD+LNxrmwHrv5hY4hrFwbhYjugw+9g@mail.gmail.com>
Date:   Tue, 8 Dec 2020 20:12:06 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Anton Blanchard <anton@...abs.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v3 2/4] membarrier: Add an actual barrier before rseq_preempt()

On Thu, Dec 3, 2020 at 9:07 PM Andy Lutomirski <luto@...nel.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@...r.kernel.org

Fixes: 2a36ab717e8f ("rseq/membarrier: Add
MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ")

which is new in 5.10, so it doesn't need cc:stable if it makes 5.10.