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


From: Xiao Guangrong
Subject: Re: [Qemu-devel] [PATCH v3 2/5] util: introduce threaded workqueue
Date: Tue, 27 Nov 2018 16:31:31 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1



On 11/26/18 6:28 PM, Paolo Bonzini wrote:
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).

That's good to me. will try it. ;)



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