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

[Alex Bennée]

Mark Burton <address@hidden> writes:

>> 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?

Yeah - surely just calling mutex_lock will serialise all the users
accessing the ldst variables?

>
> Cheers
> Mark.
>
>> -- 
>> Alex Bennée
>
>
>        +44 (0)20 7100 3485 x 210
>  +33 (0)5 33 52 01 77x 210
>
>       +33 (0)603762104
>       mark.burton

-- 
Alex Bennée