[libcxx-commits] [libcxx] [libcxx] Shared Mutex no longer holds the lock when calling notify_* on gates. (PR #107876)

[via libcxx-commits]

https://github.com/Brotcrunsher created https://github.com/llvm/llvm-project/pull/107876

Holding the associated lock while calling notify_* on a condition_variable is generally considered a pessimization, as the notified thread might "instantly" wake up, notice that it can't acquire the lock, and then goes back to sleep.

**Note:** This is my very first PR to the LLVM-Project. As such, it's possible that some kind of compiler extensions exist that render this unnecessary.

>From 10f7359197f84b796ca7765c1874eb73d2b5bfbb Mon Sep 17 00:00:00 2001
From: Brotcrunsher <Brotcrunsher at hotmail.de>
Date: Mon, 9 Sep 2024 17:16:59 +0200
Subject: [PATCH] [libcxx] Shared Mutex no longer holds the lock when calling
 notify_* on gates.

Holding the associated lock while calling notify_* on a condition_variable is generally considered a pessimization, as the notified thread might "instantly" wake up, notice that it can't acquire the lock, and then goes back to sleep.
---
 libcxx/src/shared_mutex.cpp | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/libcxx/src/shared_mutex.cpp b/libcxx/src/shared_mutex.cpp
index 1a346dda027f8e..61808337369561 100644
--- a/libcxx/src/shared_mutex.cpp
+++ b/libcxx/src/shared_mutex.cpp
@@ -38,8 +38,10 @@ bool __shared_mutex_base::try_lock() {
 }
 
 void __shared_mutex_base::unlock() {
-  lock_guard<mutex> _(__mut_);
-  __state_ = 0;
+  {
+    lock_guard<mutex> _(__mut_);
+    __state_ = 0;
+  }
   __gate1_.notify_all();
 }
 
@@ -67,16 +69,20 @@ bool __shared_mutex_base::try_lock_shared() {
 }
 
 void __shared_mutex_base::unlock_shared() {
-  lock_guard<mutex> _(__mut_);
+  unique_lock<mutex> lk(__mut_);
   unsigned num_readers = (__state_ & __n_readers_) - 1;
   __state_ &= ~__n_readers_;
   __state_ |= num_readers;
   if (__state_ & __write_entered_) {
-    if (num_readers == 0)
+    if (num_readers == 0) {
+      lk.unlock();
       __gate2_.notify_one();
+    }
   } else {
-    if (num_readers == __n_readers_ - 1)
+    if (num_readers == __n_readers_ - 1) {
+      lk.unlock();
       __gate1_.notify_one();
+    }
   }
 }