linux-kernel - Re: [PATCH backport] membarrier: Explicitly sync remote cores when SYNC

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Message-ID: <X932EI8RN90rRjyP@kroah.com>
Date:   Sat, 19 Dec 2020 13:46:08 +0100
From:   Greg KH <greg@...ah.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: Re: [PATCH backport] membarrier: Explicitly sync remote cores when
 SYNC_CORE is requested

On Mon, Dec 14, 2020 at 10:00:43AM -0800, Andy Lutomirski wrote:
> commit 758c9373d84168dc7d039cf85a0e920046b17b41 upstream
> 
> membarrier() does not explicitly sync_core() remote CPUs; instead, it
> relies on the assumption that an IPI will result in a core sync.  On x86,
> 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.  While IRET does serialize, IPI return can
> schedule, thereby switching to another task in the same mm that was
> sleeping in a syscall.  The new task could then SYSRET back to usermode
> without ever executing IRET.
> 
> Make this more robust by explicitly calling sync_core_before_usermode()
> on remote cores.  (This also helps people who search the kernel tree for
> instances of sync_core() and sync_core_before_usermode() -- one might be
> surprised that the core membarrier code doesn't currently show up in a
> such a search.)
> 
> Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: stable@...r.kernel.org
> Link: https://lore.kernel.org/r/776b448d5f7bd6b12690707f5ed67bcda7f1d427.1607058304.git.luto@kernel.org
> ---
> 
> My stable membarrier series depends on commit 2a36ab717e8f
> ("rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ").  I don't
> think it makes much sense to backport that feature, so here's a backport of
> the patch that doesn't need it.

Now queued up to 5.4.y and 5.9.y, thanks.

greg k-h