Re: [Qemu-devel] [RFC PATCH] Use atomic cmpxchg to atomically check the exclusive value in a STREX

[Frederic Konrad <fred.konrad@greensocs.com>]

On 09/06/2015 15:55, Alex Bennée wrote:
> Alex Bennée <alex.bennee@linaro.org> writes:
>
>> fred.konrad@greensocs.com writes:
>>
>>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>>
>>> This mechanism replaces the existing load/store exclusive mechanism which seems
>>> to be broken for multithread.
>>> It follows the intention of the existing mechanism and stores the target address
>>> and data values during a load operation and checks that they remain unchanged
>>> before a store.
>>>
>>> In common with the older approach, this provides weaker semantics than required
>>> in that it could be that a different processor writes the same value as a
>>> non-exclusive write, however in practise this seems to be irrelevant.
> <snip>
>>>   
>>> +/* Protect cpu_exclusive_* variable .*/
>>> +__thread bool cpu_have_exclusive_lock;
>>> +QemuMutex cpu_exclusive_lock;
>>> +
>>> +inline void arm_exclusive_lock(void)
>>> +{
>>> +    if (!cpu_have_exclusive_lock) {
>>> +        qemu_mutex_lock(&cpu_exclusive_lock);
>>> +        cpu_have_exclusive_lock = true;
>>> +    }
>>> +}
>>> +
>>> +inline void arm_exclusive_unlock(void)
>>> +{
>>> +    if (cpu_have_exclusive_lock) {
>>> +        cpu_have_exclusive_lock = false;
>>> +        qemu_mutex_unlock(&cpu_exclusive_lock);
>>> +    }
>>> +}
>> I don't quite follow. If these locks are mean to be protecting access to
>> variables then how do they do that? The lock won't block if another
>> thread is currently messing with the protected values.
> Having re-read after coffee I'm still wondering why we need the
> per-thread bool? All the lock/unlock pairs are for critical sections so
> don't we just want to serialise on the qemu_mutex_lock(), what do the
> flags add apart from allowing you to next locks that shouldn't happen?
>
>
You are probably right, this might be a rest of the old approach.
There were branches so we needed to allow next locks.

Thanks,
Fred