On 02/11/2015 14:39, Denis V. Lunev wrote:
>> This is thread-safe:
>>
>> bool owner;
>>
>> qemu_mutex_lock(&r->lock);
>> owner = r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
>> qemu_mutex_unlock(&r->lock);
>>
>> return owner;
> yep, I know.
>
> But I do not want to take the lock for check.
You can use a trylock. If it fails, it is definitely safe to return false.
> IMHO it would be better to
>
> @@ -68,11 +68,16 @@ void rfifolock_lock(RFifoLock *r)
> void rfifolock_unlock(RFifoLock *r)
> {
> qemu_mutex_lock(&r->lock);
> - assert(r->nesting > 0);
> - assert(qemu_thread_is_self(&r->owner_thread));
> + assert(rfifolock_is_owner(r));
> if (--r->nesting == 0) {
> + qemu_thread_clear(&r->owner_thread);
> r->head++;
> qemu_cond_broadcast(&r->cond);
> }
> qemu_mutex_unlock(&r->lock);
> }
> +
> +bool rfifolock_is_owner(RFifoLock *r)
> +{
> + return r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
> +}
>
> which does not require lock and thread safe.
I think it requires memory barriers though. But it can be simplified:
if you clear owner_thread, you do not need to check r->nesting in
rfifolock_is_owner.
Clearing owner_thread can be done with a simple memset.
Paolo
