From 0ba521e63455d2222b61bde70abfe4fed43ee117 Mon Sep 17 00:00:00 2001 From: yzct12345 <87620833+yzct12345@users.noreply.github.com> Date: Fri, 13 Aug 2021 19:22:51 +0000 Subject: [PATCH] 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. --- src/common/threadsafe_queue.h | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/src/common/threadsafe_queue.h b/src/common/threadsafe_queue.h index ad04df8ca..8430b9778 100644 --- a/src/common/threadsafe_queue.h +++ b/src/common/threadsafe_queue.h @@ -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(); }