Re: [Qemu-devel] [PATCH v3 2/5] util: introduce threaded workqueue

[Paolo Bonzini]

On 26/11/18 09:18, Xiao Guangrong wrote:
>>
>>> +static uint64_t get_free_request_bitmap(Threads *threads,
>>> ThreadLocal *thread)
>>> +{
>>> +    uint64_t request_fill_bitmap, request_done_bitmap, result_bitmap;
>>> +
>>> +    request_fill_bitmap =
>>> atomic_rcu_read(&thread->request_fill_bitmap);
>>> +    request_done_bitmap =
>>> atomic_rcu_read(&thread->request_done_bitmap);
>>> +    bitmap_xor(&result_bitmap, &request_fill_bitmap,
>>> &request_done_bitmap,
>>> +               threads->thread_requests_nr);
>>
>> This is not wrong, but it's a big ugly. Instead, I would:
>>
>> - Introduce bitmap_xor_atomic in a previous patch
>> - Use bitmap_xor_atomic here, getting rid of the rcu reads
> 
> Hmm, however, we do not need atomic xor operation here... that should be
> slower than
> just two READ_ONCE calls.

Yeah, I'd just go with Guangrong's version.  Alternatively, add
find_{first,next}_{same,different}_bit functions (whatever subset of the
4 you need).

Paolo