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threadsafe_queue: Fix deadlock
This fixes a lost wakeup in SPSCQueue. If the reader is in just the right position, the writer's notification will be lost and this will be a problem if the writer then does something to wait on the reader. This was discovered to affect my upcoming stacktrace PR. I don't think any performance decrease will be noticeable because an uncontended mutex is smart enough to skip the syscall. This PR might also resolve some rare deadlocks but I don't know of any examples.
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@ -48,15 +48,13 @@ public:
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ElementPtr* new_ptr = new ElementPtr();
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write_ptr->next.store(new_ptr, std::memory_order_release);
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write_ptr = new_ptr;
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++size;
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const size_t previous_size{size++};
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// Acquire the mutex and then immediately release it as a fence.
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// cv_mutex must be held or else there will be a missed wakeup if the other thread is in the
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// line before cv.wait
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// TODO(bunnei): This can be replaced with C++20 waitable atomics when properly supported.
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// See discussion on https://github.com/yuzu-emu/yuzu/pull/3173 for details.
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if (previous_size == 0) {
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std::lock_guard lock{cv_mutex};
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}
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cv.notify_one();
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}
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