fallback: Use optimistic_read in read

src/imp/fallback/mod.rs

@@ -156,29 +156,9 @@ macro_rules! atomic {
 
             #[inline]
             fn read(&self, _guard: &SeqLockWriteGuard<'static>) -> $int_type {
-                // SAFETY:
-                // - The guard guarantees that we hold the lock to write.
-                // - The raw pointer is valid because we got it from a reference.
-                //
-                // Unlike optimistic_read/write, the atomic operation is not required,
-                // because we hold the lock to write so that other threads cannot
-                // perform concurrent write operations.
-                //
-                // At the hardware level, core::sync::atomic::Atomic*::load used in optimistic_read
-                // may be lowered to atomic write operations by LLVM, but it is still considered a
-                // read operation from the view of the (software) memory model, except that it is
-                // not allowed in read-only memory (due to UnsafeCell, self.v is not read-only memory).
-                // See also https://github.com/rust-lang/miri/issues/2463.
-                // (Note that the above property is about the assembly generated by inline assembly
-                // or LLVM's backend. Doing it using write operations written in normal Rust code
-                // or LLVM IR is considered UB, even if it never mutates the value. See also the
-                // above Miri issue and https://github.com/rust-lang/rust/issues/32976#issuecomment-446775360)
-                //
-                // Also, according to atomic-memcpy's asm test, there seems
-                // to be no tier 1 or tier 2 platform that generates such code
-                // for a pointer-width relaxed load + acquire fence:
-                // https://github.com/taiki-e/atomic-memcpy/tree/v0.1.3/tests/asm-test/asm
-                unsafe { self.v.get().read() }
+                // This calls optimistic_read that can return teared value, but the resulting value
+                // is guaranteed not to be teared because we hold the lock to write.
+                self.optimistic_read()
             }
 
             #[inline]