linux-kernel - Re: [PATCH] percpu: make this_cpu_generic

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Message-ID: <20170925154404.GA560070@devbig577.frc2.facebook.com>
Date:   Mon, 25 Sep 2017 08:44:04 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Christoph Lameter <cl@...ux.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Pranith Kumar <bobby.prani@...il.com>,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH] percpu: make this_cpu_generic_read() atomic w.r.t.
 interrupts

Hello,

On Mon, Sep 25, 2017 at 04:33:02PM +0100, Mark Rutland wrote:
> Unfortunately, the generic this_cpu_read(), which is intended to be
> irq-safe, is not:
> 
> #define this_cpu_generic_read(pcp)                                      \
> ({                                                                      \
>         typeof(pcp) __ret;                                              \
>         preempt_disable_notrace();                                      \
>         __ret = raw_cpu_generic_read(pcp);                              \
>         preempt_enable_notrace();                                       \
>         __ret;                                                          \
> })

I see.  Yeah, that looks like the bug there.

> I guess it'd be preferable to manipulate that in-place.
> 
> > Adding READ_ONCE() doesn't generically guarantee that the reads won't
> > be split - e.g. there are arch which simply can't load a 64bit value
> > with a single instruction.
> 
> In which case, it really sounds like this_cpu_generic_read() needs to
> disable interrupts too...

Can you please spin up a patch for this?

Thanks.

-- 
tejun