Fix #697

1. Put fences before and after the intrinsics (it would be bad if the
   disable interrupt operation was moved before the primask read).
2. Remove the `nomem` attribute, so that LLVM believes the asm blocks
   touch memory and hence cannot move across the fences.

Author: jonathanpallant

cortex-m/src/critical_section.rs

@@ -12,7 +12,8 @@ unsafe impl Impl for SingleCoreCriticalSection {
         // u32 instead of using the primask::read() function to minimize the number of processor
         // cycles during which interrupts are disabled.
         let restore_state = primask::read_raw();
-        // NOTE: Fence guarantees are provided by interrupt::disable(), which performs a `compiler_fence(SeqCst)`.
+        // NOTE: Fence guarantees are provided by interrupt::disable(),
+        // which performs a `compiler_fence(SeqCst)` fore and aft.
         interrupt::disable();
         restore_state
     }

cortex-m/src/interrupt.rs

@@ -41,7 +41,10 @@ unsafe impl<T: Nr + Copy> InterruptNumber for T {
 #[inline]
 #[asm_cfg(cortex_m)]
 pub fn disable() {
-    unsafe { asm!("cpsid i", options(nomem, nostack, preserves_flags)) };
+    // Ensure no preceeding memory accesses are reordered to after interrupts are disabled.
+    compiler_fence(Ordering::SeqCst);
+
+    unsafe { asm!("cpsid i", options(nostack, preserves_flags)) };
 
     // Ensure no subsequent memory accesses are reordered to before interrupts are disabled.
     compiler_fence(Ordering::SeqCst);
@@ -58,7 +61,10 @@ pub unsafe fn enable() {
     // Ensure no preceeding memory accesses are reordered to after interrupts are enabled.
     compiler_fence(Ordering::SeqCst);
 
-    unsafe { asm!("cpsie i", options(nomem, nostack, preserves_flags)) };
+    unsafe { asm!("cpsie i", options(nostack, preserves_flags)) };
+
+    // Ensure no subsequent memory accesses are reordered to before interrupts are disabled.
+    compiler_fence(Ordering::SeqCst);
 }
 
 /// Execute closure `f` in an interrupt-free context.

cortex-m/src/register/primask.rs

@@ -64,7 +64,9 @@ pub fn read_raw() -> u32 {
 pub unsafe fn write_raw(r: u32) {
     // Ensure no preceeding memory accesses are reordered to after interrupts are possibly enabled.
     compiler_fence(Ordering::SeqCst);
-    unsafe { asm!("msr PRIMASK, {}", in(reg) r, options(nomem, nostack, preserves_flags)) };
+
+    unsafe { asm!("msr PRIMASK, {}", in(reg) r, options(nostack, preserves_flags)) };
+
     // Ensure no subsequent memory accesses are reordered to before interrupts are possibly disabled.
     compiler_fence(Ordering::SeqCst);
 }