[libcxx] Don't hold the lock when calling notify_* on gates in std::shared_mutex (llvm#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.

Author: Brotcrunsher

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();
+    }
   }
 }