Rollup merge of rust-lang#41730 - bholley:arc_comment, r=aturon

Document the reasoning for the Acquire/Release handshake when dropping Arcs.

Split out from rust-lang#41714. r? @aturon

src/liballoc/arc.rs

@@ -767,7 +767,18 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
         // > through this reference must obviously happened before), and an
         // > "acquire" operation before deleting the object.
         //
+        // In particular, while the contents of an Arc are usually immutable, it's
+        // possible to have interior writes to something like a Mutex<T>. Since a
+        // Mutex is not acquired when it is deleted, we can't rely on its
+        // synchronization logic to make writes in thread A visible to a destructor
+        // running in thread B.
+        //
+        // Also note that the Acquire fence here could probably be replaced with an
+        // Acquire load, which could improve performance in highly-contended
+        // situations. See [2].
+        //
         // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
+        // [2]: (https://github.com/rust-lang/rust/pull/41714)
         atomic::fence(Acquire);
 
         unsafe {