linux-kernel - Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless

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Message-ID: <CACVXFVMuhFoPat7FAznr4ji5vXeeVBNCWya=JdYGLYqBv5SFUQ@mail.gmail.com>
Date:	Tue, 12 Mar 2013 12:03:23 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

On Tue, Mar 12, 2013 at 11:39 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> Atomic operations that return a value are required to act as full memory
> barriers.  This means that code relying on ordering provided by these
> atomic operations must also do ordering, either by using an explicit
> memory barrier or by relying on guarantees from atomic operations.
>
> For example:
>
>         CPU 0                                   CPU 1
>
>         X = 1;                                  r1 = Z;
>         if (atomic_inc_unless_negative(&Y)      smp_mb();
>                 do_something();
>         Z = 1;                                  r2 = X;
>
> Assuming X and Z are initially zero, if r1==1, we are guaranteed
> that r2==1.  However, CPU 1 needs its smp_mb() in order to pair with
> the barrier implicit in atomic_inc_unless_negative().
>
> Make sense?

Yes, it does, and thanks for the explanation.

But looks the above example is not what Frederic described:

"the above atomic_read() might return -1 because there is no
guarantee it's seeing the recent update on the remote CPU."

Even I am not sure if adding one smp_mb() around atomic_read()
can guarantee that too.

Andrew, please ignore the patch, thanks.

Thanks,
-- 
Ming Lei
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