Re: [Xen-devel] [PATCH v2 2/3] xen/atomic: Implement atomic_{inc, dec}_bounded()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/07/14 15:05, Jan Beulich wrote:
>>>> On 02.07.14 at 15:47, <andrew.cooper3@xxxxxxxxxx> wrote:
>> These will increment/decremented an atomic_t, while ensuring that the 
>> atomic_t
>> doesn't hit a specific bound.
>>
>> This involves introducing atomic_cmpxchg() for x86, which previously only
>> existed on ARM.
>>
>> Suggested-by: Don Slutz <dslutz@xxxxxxxxxxx>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> CC: Keir Fraser <keir@xxxxxxx>
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
> You should be getting used to Cc all "REST" maintainers on respective
> changes.

Oops yes - I will see about updating my auto-cc list hooks.

>
>> --- /dev/null
>> +++ b/xen/common/atomic.c
>> @@ -0,0 +1,35 @@
>> +#include <xen/atomic.h>
>> +
>> +bool_t atomic_inc_bounded(atomic_t *v, int bound)
>> +{
>> +    int old, new, prev = atomic_read(v);
>> +
>> +    do
>> +    {
>> +        old = prev;
>> +        new = old + 1;
>> +        if ( new >= bound )
>> +            return 0;
>> +
>> +        prev = atomic_cmpxchg(v, old, new);
>> +    } while ( prev != old );
>> +
>> +    return 1;
>> +}
>> +
>> +bool_t atomic_dec_bounded(atomic_t *v, int bound)
>> +{
>> +    int old, new, prev = atomic_read(v);
>> +
>> +    do
>> +    {
>> +        old = prev;
>> +        new = old - 1;
>> +        if ( new <= bound )
>> +            return 0;
>> +
>> +        prev = atomic_cmpxchg(v, old, new);
>> +    } while ( prev != old );
>> +
>> +    return 1;
>> +}
> Same question here: Do we really need these? There are various uses
> of cmpxchg() in common code already, and in patch 3 you don't really
> need the cmpxchg to happen on an atomic_t, so plain cmpxchg() with
> the little bit of extra logic open coded would seem fine to me.
>
> Jan
>

These as in atomic_{inc,dec}_bounded() ?

They logically fit alongside atomic_{inc,dec}_and_test(), and I came to
the conclusion that they would likely be useful elsewhere.

I realise that these are not fantastic reasons alone...

~Andrew

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