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

["Alex Bennée" <alex.bennee@linaro.org>]

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?


-- 
Alex Bennée