> On 9 Jun 2015, at 15:55, Alex Bennée <alex.ben...@linaro.org> wrote:
>
>
> Alex Bennée <alex.ben...@linaro.org> writes:
>
>> fred.kon...@greensocs.com writes:
>>
>>> From: KONRAD Frederic <fred.kon...@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 mean the “cpu_have_exclusive_lock” bools?
Cheers
Mark.
> --
> Alex Bennée
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mark.burton
