Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

[Ming Lei]

On Tue, Mar 12, 2013 at 7:59 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 2013/3/9 Ming Lei <tom.leiming@xxxxxxxxx>:
>> 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@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Shaohua Li <shli@xxxxxxxxxx>
>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
>> ---
>>  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.

Yes, you are right. Also looks memory barrier is needed around
atomic_inc() too.

But I have a question, why a memory barrier can guarantee that
remote CPU can see the recent update?  I understand that memory
barrier only orders consecutive memory access, and but here
not see this kind of pattern. Sorry for a possibly stupid question.


Thanks,
-- 
Ming Lei
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