linux-kernel - Re: [PATCH v3 1/4] x86/membarrier: Get rid of a dubious optimization

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Message-ID: <CALCETrULOhzyTjd+K5ByeRZ2Sn2-EjpaMJd2DosZ-V1X5DUojA@mail.gmail.com>
Date:   Tue, 8 Dec 2020 20:10:17 -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 1/4] x86/membarrier: Get rid of a dubious optimization

On Thu, Dec 3, 2020 at 9:07 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> sync_core_before_usermode() had an incorrect optimization.  If we're
> in an IRQ, we can get to usermode without IRET -- we just have to
> schedule to a different task in the same mm and do SYSRET.
> Fortunately, there were no callers of sync_core_before_usermode()
> that could have had in_irq() or in_nmi() equal to true, because it's
> only ever called from the scheduler.
>
> While we're at it, clarify a related comment.
>

Fixes: ac1ab12a3e6e ("lockin/x86: Implement sync_core_before_usermode()")

> Cc: stable@...r.kernel.org
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>