linux-kernel - Re: [tip: x86/urgent] membarrier: Add an actual barrier before rseq

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Message-ID: <CALCETrW4KAbvAE45T4N0CMdR7XQyrRQTEAq5fX=xVpGAEb3KwA@mail.gmail.com>
Date:   Mon, 14 Dec 2020 10:05:18 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>
Cc:     linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        stable <stable@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: [tip: x86/urgent] membarrier: Add an actual barrier before rseq_preempt()

On Wed, Dec 9, 2020 at 12:42 AM tip-bot2 for Andy Lutomirski
<tip-bot2@...utronix.de> wrote:
>
> The following commit has been merged into the x86/urgent branch of tip:
>
> Commit-ID:     2ecedd7569080fd05c1a457e8af2165afecfa29f
> Gitweb:        https://git.kernel.org/tip/2ecedd7569080fd05c1a457e8af2165afecfa29f
> Author:        Andy Lutomirski <luto@...nel.org>
> AuthorDate:    Thu, 03 Dec 2020 21:07:04 -08:00
> Committer:     Thomas Gleixner <tglx@...utronix.de>
> CommitterDate: Wed, 09 Dec 2020 09:37:43 +01:00
>
> membarrier: Add an actual barrier before rseq_preempt()
>
> It seems 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.
>
> Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")

Whoops, this got mangled on its way to tip.  This should be:

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

and this patch does not need to be backported.