Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Mar 09, 2013 at 11:38:21PM +0800, Ming Lei wrote:
> 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@linux-foundation.org>
> Cc: Shaohua Li <shli@kernel.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Ming Lei <tom.leiming@gmail.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))

As Frederic noted, you need an smp_mb() right here.  Because
atomic_inc_unless_negative() returns a value, it is required to
act as a full memory barrier.  On the other code paths, atomic_cmpxchg()
supplies the needed memory ordering, but not on this path.

> +			return 0;
> +		t = atomic_cmpxchg(p, v, v + 1);
> +		if (likely(t == v))
>  			return 1;
> +		v = t;
>  	}
> -	return 0;
>  }
>  #endif
>  
>  #ifndef atomic_dec_unless_positive
>  static inline int atomic_dec_unless_positive(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))

Ditto here.

							Thanx, Paul

> +			return 0;
> +		t = atomic_cmpxchg(p, v, v - 1);
> +		if (likely(t == v))
>  			return 1;
> +		v = t;
>  	}
> -	return 0;
>  }
>  #endif
>  
> -- 
> 1.7.9.5
> 
> --
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