On 06/18/2013 01:36 AM, Paolo Bonzini wrote:
>> Why all the ifdefs? If __atomic support is present, then __ATOMIC defines
>> will
>> exist.
>
> Then I can just use "#ifdef __ATOMIC_RELAXED" instead of
> "#if QEMU_GNUC_PREREQ(4, 8)"?
I'd say so.
> I have no idea, but I can say which semantics I want:
>
> 1) Linux kernel memory barrier semantics for smp_*mb (i.e. express the
> barriers
> in terms of read/write/full, not in terms of acq/rel/seqcst);
>
> 2) Java volatile semantics for atomic_mb_*.
>
> Basically, I cannot claim I understand this stuff 100%, but at least I could
> use sources I trust to implement it.
Fair enough. Excellent pointers to have in the documentation, anyhow.
>>> +#ifndef atomic_read
>>> +#define atomic_read(ptr) (*(__typeof__(*ptr) *volatile) (ptr))
>>> #endif
>>>
>>> +#ifndef atomic_set
>>> +#define atomic_set(ptr, i) ((*(__typeof__(*ptr) *volatile) (ptr)) =
>>> (i))
>>> +#endif
>>
>> Use
>>
>> __atomic_load(..., __ATOMIC_RELAXED)
>> __atomic_store(..., __ATOMIC_RELAXED)
>>
>> ?
>
> Same here, I didn't want proliferation of #ifdefs beyond what is actually
> required.
Not knowing exactly where these might be used within the code base, I'd be
worried about someone applying them to a uint64_t, somewhere a 32-bit host
might see it. At which point the above is going to be silently wrong, loaded
with two 32-bit pieces.
Given that we're not requiring gcc 4.8, and cannot guarantee use of
__atomic_load, perhaps we ought to do something like
#define atomic_read(ptr) \
({ if (sizeof(*(ptr)) > sizeof(ptr)) invalid_atomic_read(); \
*(__typeof__(*ptr) *volatile) (ptr)); })
which should generate a link error when reading a size we can't guarantee will
Just Work.
> The FAQ also has an "important note", however:
>
> Important Note: Note that it is important for both threads to access
> the same volatile variable in order to properly set up the happens-before
> relationship. It is not the case that everything visible to thread A
> when it writes volatile field f becomes visible to thread B after it
> reads volatile field g. The release and acquire have to "match" (i.e.,
> be performed on the same volatile field) to have the right semantics.
>
> Is this final "important note" the difference between ACQ_REL and SEQ_CST?
Yes, exactly.
r~
