Add random failures to compare_exchange_weak

src/data_race.rs

@@ -75,7 +75,7 @@ use rustc_target::abi::Size;
 
 use crate::{
     ImmTy, Immediate, InterpResult, MPlaceTy, MemPlaceMeta, MiriEvalContext, MiriEvalContextExt,
-    OpTy, Pointer, RangeMap, ScalarMaybeUninit, Tag, ThreadId, VClock, VTimestamp,
+    OpTy, Pointer, RangeMap, Scalar, ScalarMaybeUninit, Tag, ThreadId, VClock, VTimestamp,
     VectorIdx, MemoryKind, MiriMemoryKind
 };
 
@@ -544,7 +544,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriEvalContextExt<'mir, 'tcx> {
 
     /// Perform an atomic compare and exchange at a given memory location.
     /// On success an atomic RMW operation is performed and on failure
-    /// only an atomic read occurs.
+    /// only an atomic read occurs. If `can_fail_spuriously` is true,
+    /// then we treat it as a "compare_exchange_weak" operation, and
+    /// some portion of the time fail even when the values are actually
+    /// identical.
     fn atomic_compare_exchange_scalar(
         &mut self,
         place: MPlaceTy<'tcx, Tag>,
@@ -552,7 +555,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriEvalContextExt<'mir, 'tcx> {
         new: ScalarMaybeUninit<Tag>,
         success: AtomicRwOp,
         fail: AtomicReadOp,
+        can_fail_spuriously: bool,
     ) -> InterpResult<'tcx, Immediate<Tag>> {
+        use rand::Rng as _;
         let this = self.eval_context_mut();
 
         // Failure ordering cannot be stronger than success ordering, therefore first attempt
@@ -560,15 +565,21 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriEvalContextExt<'mir, 'tcx> {
         // read ordering and write in the success case.
         // Read as immediate for the sake of `binary_op()`
         let old = this.allow_data_races_mut(|this| this.read_immediate(place.into()))?;
-
         // `binary_op` will bail if either of them is not a scalar.
         let eq = this.overflowing_binary_op(mir::BinOp::Eq, old, expect_old)?.0;
-        let res = Immediate::ScalarPair(old.to_scalar_or_uninit(), eq.into());
+        // If the operation would succeed, but is "weak", fail 50% of the time.
+        // FIXME: this is quite arbitrary.
+        let cmpxchg_success = eq.to_bool()?
+            && (!can_fail_spuriously || this.memory.extra.rng.borrow_mut().gen_range(0, 2) == 0);
+        let res = Immediate::ScalarPair(
+            old.to_scalar_or_uninit(),
+            Scalar::from_bool(cmpxchg_success).into(),
+        );
 
         // Update ptr depending on comparison.
         // if successful, perform a full rw-atomic validation
         // otherwise treat this as an atomic load with the fail ordering.
-        if eq.to_bool()? {
+        if cmpxchg_success {
             this.allow_data_races_mut(|this| this.write_scalar(new, place.into()))?;
             this.validate_atomic_rmw(place, success)?;
         } else {

src/shims/intrinsics.rs

@@ -518,9 +518,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         Ok(())
     }
 
-    fn atomic_compare_exchange(
+    fn atomic_compare_exchange_impl(
         &mut self, args: &[OpTy<'tcx, Tag>], dest: PlaceTy<'tcx, Tag>,
-        success: AtomicRwOp, fail: AtomicReadOp
+        success: AtomicRwOp, fail: AtomicReadOp, can_fail_spuriously: bool
     ) -> InterpResult<'tcx> {
         let this = self.eval_context_mut();
 
@@ -538,7 +538,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         
         let old = this.atomic_compare_exchange_scalar(
-            place, expect_old, new, success, fail
+            place, expect_old, new, success, fail, can_fail_spuriously
         )?;
 
         // Return old value.
@@ -546,14 +546,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         Ok(())
     }
 
+    fn atomic_compare_exchange(
+        &mut self, args: &[OpTy<'tcx, Tag>], dest: PlaceTy<'tcx, Tag>,
+        success: AtomicRwOp, fail: AtomicReadOp
+    ) -> InterpResult<'tcx> {
+        self.atomic_compare_exchange_impl(args, dest, success, fail, false)
+    }
+
     fn atomic_compare_exchange_weak(
         &mut self, args: &[OpTy<'tcx, Tag>], dest: PlaceTy<'tcx, Tag>,
         success: AtomicRwOp, fail: AtomicReadOp
     ) -> InterpResult<'tcx> {
-
-        // FIXME: the weak part of this is currently not modelled,
-        //  it is assumed to always succeed unconditionally.
-        self.atomic_compare_exchange(args, dest, success, fail)
+        self.atomic_compare_exchange_impl(args, dest, success, fail, true)
     }
 
     fn float_to_int_unchecked<F>(

tests/run-pass/atomic.rs

@@ -24,7 +24,20 @@ fn atomic_bool() {
         assert_eq!(*ATOMIC.get_mut(), false);
     }
 }
-
+// There isn't a trait to use to make this generic, so just use a macro
+macro_rules! compare_exchange_weak_loop {
+    ($atom:expr, $from:expr, $to:expr, $succ_order:expr, $fail_order:expr) => {
+        loop {
+            match $atom.compare_exchange_weak($from, $to, $succ_order, $fail_order) {
+                Ok(n) => {
+                    assert_eq!(n, $from);
+                    break;
+                }
+                Err(n) => assert_eq!(n, $from),
+            }
+        }
+    };
+}
 fn atomic_isize() {
     static ATOMIC: AtomicIsize = AtomicIsize::new(0);
 
@@ -40,11 +53,11 @@ fn atomic_isize() {
     ATOMIC.compare_exchange(0, 1, SeqCst, SeqCst).ok();
 
     ATOMIC.store(0, SeqCst);
-
-    assert_eq!(ATOMIC.compare_exchange_weak(0, 1, Relaxed, Relaxed), Ok(0));
+    compare_exchange_weak_loop!(ATOMIC, 0, 1, Relaxed, Relaxed);
     assert_eq!(ATOMIC.compare_exchange_weak(0, 2, Acquire, Relaxed), Err(1));
     assert_eq!(ATOMIC.compare_exchange_weak(0, 1, Release, Relaxed), Err(1));
-    assert_eq!(ATOMIC.compare_exchange_weak(1, 0, AcqRel, Relaxed), Ok(1));
+    compare_exchange_weak_loop!(ATOMIC, 1, 0, AcqRel, Relaxed);
+    assert_eq!(ATOMIC.load(Relaxed), 0);
     ATOMIC.compare_exchange_weak(0, 1, AcqRel, Relaxed).ok();
     ATOMIC.compare_exchange_weak(0, 1, SeqCst, Relaxed).ok();
     ATOMIC.compare_exchange_weak(0, 1, Acquire, Acquire).ok();
@@ -58,10 +71,7 @@ fn atomic_u64() {
 
     ATOMIC.store(1, SeqCst);
     assert_eq!(ATOMIC.compare_exchange(0, 0x100, AcqRel, Acquire), Err(1));
-    assert_eq!(
-        ATOMIC.compare_exchange_weak(1, 0x100, AcqRel, Acquire),
-        Ok(1)
-    );
+    compare_exchange_weak_loop!(ATOMIC, 1, 0x100, AcqRel, Acquire);
     assert_eq!(ATOMIC.load(Relaxed), 0x100);
 }