Rollup merge of rust-lang#96619 - akiekintveld:same_mutex_check_relaxed_ordering, r=m-ou-se

Relax memory ordering used in SameMutexCheck

`SameMutexCheck` only requires atomicity for `self.addr`, but does not need ordering of other memory accesses in either the success or failure case. Using `Relaxed`, the code still correctly handles the case when two threads race to store an address.

Author: JohnTitor

library/std/src/sys_common/condvar/check.rs

@@ -24,8 +24,14 @@ impl SameMutexCheck {
     }
     pub fn verify(&self, mutex: &MovableMutex) {
         let addr = mutex.raw() as *const imp::Mutex as *const () as *mut _;
-        match self.addr.compare_exchange(ptr::null_mut(), addr, Ordering::SeqCst, Ordering::SeqCst)
-        {
+        // Relaxed is okay here because we never read through `self.addr`, and only use it to
+        // compare addresses.
+        match self.addr.compare_exchange(
+            ptr::null_mut(),
+            addr,
+            Ordering::Relaxed,
+            Ordering::Relaxed,
+        ) {
             Ok(_) => {}               // Stored the address
             Err(n) if n == addr => {} // Lost a race to store the same address
             _ => panic!("attempted to use a condition variable with two mutexes"),