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

[Mark Burton]

> On 9 Jun 2015, at 15:55, Alex Bennée <address@hidden> wrote:
> 
> 
> Alex Bennée <address@hidden> writes:
> 
>> address@hidden writes:
>> 
>>> From: KONRAD Frederic <address@hidden>
>>> 
>>> 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