[PATCH] malloc: Use correct C11 atomics for fastbin

[Wilco Dijkstra]

Hi,

>> Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org> writes:
>>> +/* Atomically pop from the fastbin list.  The arena lock must be held to
>>> +   block other threads removing entries, avoiding the ABA issue.  */
>>
>> If the arena lock must be held anyway, why go through the compare and
>> exchange overhead?  We know we're the only thread accessing it.
>
> Other threads are adding entries without the arena lock.

Yes, malloc is blocking but free isn't and accesses the freelist concurrently.
It's a really weird design. Splitting the free list into a local one and a shared
one would be far better - no atomics when you have the malloc lock, and if
the local free list is empty it takes one atomic to copy all shared entries.

Cheers,
Wilco