Thread safety of AAs

[Alex Rønne Petersen]

On 16-05-2012 05:21, H. S. Teoh wrote:
> On Wed, May 16, 2012 at 05:06:54AM +0200, Alex Rønne Petersen wrote:
>> On 16-05-2012 05:03, H. S. Teoh wrote:
>>> On Wed, May 16, 2012 at 04:35:17AM +0200, Alex Rønne Petersen wrote:
>>>> Hi,
>>>>
>>>> Suppose that I have an AA that I'm doing lookups on from one thread,
>>>> and writing to in another. Is this safe at all? Naturally, I'm
>>>> willing to accept the data races involved, but the question is
>>>> whether the concurrent lookup + mutation is guaranteed to be safe.
>>> [...]
>>>
>>> Safe as in, no memory corruption? Or safe as in, the data will be
>>> consistent (barring any data races)?
>>
>> As in no memory corruption.
> [...]
>
> Hmm. Just noticed that the current aaA.d, in _aaDelX, after a slot is
> removed from the linked list gc_free is called on the slot. IIRC, if the
> mutator calls gc_free while the reader holds a reference to the slot,
> you may be accessing invalid memory. (E.g., reader looks up key being
> deleted, gets the pointer to that slot before the mutator does, then the
> CPU context-switches to the mutator, which calls gc_free, which cleans
> up that slot, now the reader has an invalid pointer.)
>
> I don't know if this will lead to memory corruption, but it sure looks
> dangerous to me.

See, this is why explicit deallocation of GC memory is bad. ;)

I guess I might just resort to using an R/W mutex.

>
>
>>> Memory safety I'm not sure, I _think_ it might be safe, but I have my
>>> doubts; data consistency, likely not, because you could potentially be
>>> reading partially-copied data (say the mutator was assigning new data to
>>> an existing key and the reader is reading that same data
>>> simultaneously; you may be seeing a partial copy of the new data
>>> intermixed with the old data).
>>
>> Assuming the AA implementation only does aligned reads/writes, there
>> should be no problem with word tearing on any modern architecture.
>> But I don't know if it does that...
> [...]
>
> If your data is larger than a word, you'd still have a problem, though.
>
>
> T
>

It's OK in my case, since I'm just storing a pointer.

-- 
- Alex