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

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Message-ID: <20170927101019.GB32150@leverpostej>
Date:   Wed, 27 Sep 2017 11:10:19 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christopher Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Pranith Kumar <bobby.prani@...il.com>,
        linux-arch@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] percpu: make this_cpu_generic_read() atomic w.r.t.
 interrupts

On Wed, Sep 27, 2017 at 11:01:13AM +0200, Thomas Gleixner wrote:
> The default for 32bit architectures is that they CANNOT do atomic
> fetch/write of 8 bytes.

Indeed.

In addition, even the native word size accesses weren't guaranteed to be
atomic, as we hadn't used the appropriate accessors to prevent the
compiler from messing with things behind our backs.

> So instead of forcing that to be implemented in every affected
> architecture this wants to be addressed in generic code and those few
> 32bit architectures which can do atomic double word fetch/write
> implement the magic functions to do so.

I've done this in v2 [1], which Tejun has queued [2] as a fix for v4.14.

Thanks,
Mark.

[1] https://lkml.kernel.org/r/1506426112-19826-1-git-send-email-mark.rutland@arm.com
[2] https://lkml.kernel.org/r/20170926144118.GA70381@devbig577.frc2.facebook.com