Create mutex-locking-and-unlocking.md

initial version of "Create mutex-locking-and-unlocking.md"

src/arc-mutex/mutex-locking-and-unlocking.md

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+# Locking and Unlocking
+Locking and unlocking is simple with our layout; Make sure the mutex is in the correct state, then toggle it.
+
+    use core::sync::fence;
+     
+    impl<T> Mutex<T>{
+        pub fn lock(&self)->MutexGuard<T>{
+            while self.is_locked.compare_exchange(
+                false,//expecting it to not be locked
+                true ,//lock it if it is not already locked
+                Ordering::Acquire,//If we succeed, use acquire ordering to prevent 
+                Ordering::Release//If we faill to acquire the lock, it does not matter which ordering we choose, so choose the fastest.
+            ).is_err(){}//If we fail to aquire the lock immediately try again.
+            
+            fence(Ordering::Acquire);//prevent writes to the data protected by the mutex from being reordered before the mutex is actually acquired.
+            //safety:The previous steps we took means that no one else has the lock
+            unsafe{return &mut self.value.get()}
+        }
+        
+        ///Safety:Make sure the caller has acquired the lock, and will not dereference the return mutable reference again.
+        pub unsafe fn unlock(&self){
+            if self.is_locked.compare_exchange(
+                true,//The caller should ensure the mutex is locked
+                false,//Mark the lock as unlocked
+                Ordering::Release,
+                Ordering::Relaxed,//We are about to panic anyway, the ordering doesn't matter
+            ).is_err{
+                panic!("Unlock called while the lock was not locked")
+            };
+        }
+    }
+
+ 
+
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