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threadsafe_queue: Fix deadlock

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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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yzct12345 2021-08-13 19:22:51 +00:00 committed by GitHub
parent 71d8d84b59
commit 0ba521e634
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1 changed files with 4 additions and 6 deletions
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10
src/common/threadsafe_queue.h
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@ -46,15 +46,13 @@ public:
ElementPtr* new_ptr = new ElementPtr();
write_ptr->next.store(new_ptr, std::memory_order_release);
write_ptr = new_ptr;
++size;
const size_t previous_size{size++};
// Acquire the mutex and then immediately release it as a fence.
// cv_mutex must be held or else there will be a missed wakeup if the other thread is in the
// line before cv.wait
// TODO(bunnei): This can be replaced with C++20 waitable atomics when properly supported.
// See discussion on https://github.com/yuzu-emu/yuzu/pull/3173 for details.
if (previous_size == 0) {
std::lock_guard lock{cv_mutex};
}
std::lock_guard lock{cv_mutex};
cv.notify_one();
}