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

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Date:	Tue, 12 Mar 2013 00:59:59 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

2013/3/9 Ming Lei <tom.leiming@...il.com>:
> Generally, both atomic_inc_unless_negative() and
> atomic_dec_unless_positive() need at least two atomic_cmpxchg()
> to complete the atomic operation. In fact, the 1st atomic_cmpxchg()
> is just used to read current value of the atomic variable at most times.
>
> Considered memory barrier, bus lock, cache walking, etc. things may be
> involved in atomic_cmpxchg(), it is much expensive than atomic_read(),
> which is just the simple below:
>
>         (*(volatile int *)&(v)->counter)
>
> so this patch can save one extra atomic_cmpxchg() for the two
> helpers under general situation, and should improve them a bit.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Shaohua Li <shli@...nel.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Ming Lei <tom.leiming@...il.com>
> ---
>  include/linux/atomic.h |   28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> index 5b08a85..aa951d8 100644
> --- a/include/linux/atomic.h
> +++ b/include/linux/atomic.h
> @@ -63,26 +63,34 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
>  #ifndef atomic_inc_unless_negative
>  static inline int atomic_inc_unless_negative(atomic_t *p)
>  {
> -       int v, v1;
> -       for (v = 0; v >= 0; v = v1) {
> -               v1 = atomic_cmpxchg(p, v, v + 1);
> -               if (likely(v1 == v))
> +       int v, t;
> +
> +       v = atomic_read(p);
> +       while (1) {
> +               if (unlikely(v < 0))
> +                       return 0;

But atomic_read() lacks the full memory barrier that is needed for
proper atomicity here.

For example if the initial value of p is -1 and another CPU just did
an atomic_inc() that resulted in the new value to be 0, the above
atomic_read() might return -1 because there is no guarantee it's
seeing the recent update on the remote CPU.

atomic_cmpxchg() OTOH provides that guarantee.
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