Re: [PATCH 9/9] docs/style: call out the use of GUARD macros

[Alex Bennée]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> On 20.04.23 18:57, Alex Bennée wrote:
>> There use makes our code safer so we should mention them.
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>
>
>> ---
>>   docs/devel/style.rst | 36 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 36 insertions(+)
>> diff --git a/docs/devel/style.rst b/docs/devel/style.rst
>> index 0bd01f3fca..b50a981a86 100644
>> --- a/docs/devel/style.rst
>> +++ b/docs/devel/style.rst
>> @@ -657,6 +657,42 @@ that declaration and the new code.
>>     See :ref:`qom` for more details.
>>   +QEMU GUARD macros
>> +=================
>> +
>> +QEMU provides a number of ``_GUARD`` macros intended to make the
>> +handling of multiple exit paths easier. For example using
>> +``QEMU_LOCK_GUARD`` to take a lock will ensure the lock is released on
>> +exit from the function.
>> +
>> +.. code-block:: c
>> +
>> +    static int my_critical_function(SomeState *s, void *data)
>> +    {
>> +        QEMU_LOCK_GUARD(&s->lock);
>> +        do_thing1(data);
>> +        if (check_state2(data)) {
>> +            return -1;
>> +        }
>> +        do_thing3(data);
>> +        return 0;
>> +    }
>
> For more clearness, maybe add an equivalent code with qemu_mutex_lock() / 
> qemu_mutex_unlock(), I mean:
>
> The equivalent code without _GUARD macro makes us to carefully put 
> qemu_mutex_unlock() on all exit points:
>
> .. code-block:: c
>
>     static int my_critical_function(SomeState *s, void *data)
>     {
>         qemu_mutex_lock(&s->lock);
>         do_thing1(data);
>         if (check_state2(data)) {
>             qemu_mutex_unlock(&s->lock);
>             return -1;
>         }
>         do_thing3(data);
>         qemu_mutex_unlock(&s->lock);
>         return 0;
>     }
>
>> +
>> +will ensure s->lock is released however the function is exited. There
>> +are often ``WITH_`` forms of macros which more easily wrap around a
>> +block inside a function.
>> +
>> +.. code-block:: c
>> +
>> +    WITH_RCU_READ_LOCK_GUARD() {
>> +        QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
>> +            err = do_the_thing(kid->child);
>> +            if (err < 0) {
>> +                return err;
>> +            }
>> +        }
>> +    }
>> +
>
> and maybe similar here.

I added the example although I didn't repeat it for the WITH form
because readers should hopefully have understood the idea with the first
example.


>
>>   Error handling and reporting
>>   ============================
>>   


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro