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

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Message-ID: <20170925151826.GK828415@devbig577.frc2.facebook.com>
Date:   Mon, 25 Sep 2017 08:18:27 -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, Mark.

On Mon, Sep 25, 2017 at 02:24:32PM +0100, Mark Rutland wrote:
> As raw_cpu_generic_read() is a plain read from a raw_cpu_ptr() address,
> it's possible (albeit unlikely) that the compiler will split the access
> across multiple instructions.
> 
> In this_cpu_generic_read() we disable preemption but not interrupts
> before calling raw_cpu_generic_read(). Thus, an interrupt could be taken
> in the middle of the split load instructions. If a this_cpu_write() or
> RMW this_cpu_*() op is made to the same variable in the interrupt
> handling path, this_cpu_read() will return a torn value.
> 
> Avoid this by using READ_ONCE() to inhibit tearing.

That's why there are irq-safe variants of the operations.  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.

Thanks.

-- 
tejun