Apply review changes, incrementing the clocks twice is an unnecessary hold-over from earlier versions so fixed.

src/data_race.rs

@@ -37,6 +37,24 @@
 //! to a acquire load and a release store given the global sequentially consistent order
 //! of the schedule.
 //!
+//! The timestamps used in the data-race detector assign each sequence of non-atomic operations
+//! followed by a single atomic or concurrent operation a single timestamp.
+//! Write, Read, Write, ThreadJoin will be represented by a single timestamp value on a thread
+//! This is because extra increment operations between the operations in the sequence are not
+//! required for accurate reporting of data-race values.
+//!
+//! If the timestamp was not incremented after the atomic operation, then data-races would not be detected:
+//!  Example - this should report a data-race but does not:
+//!   t1: (x,0), atomic[release A],                    t1=(x+1, 0  ), write(var B),
+//!   t2: (0,y)                   , atomic[acquire A], t2=(x+1, y+1),             ,write(var B)
+//!
+//! The timestamp is not incremented before an atomic operation, since the result is indistinguishable
+//! from the value not being incremented.
+//!    t: (x, 0), atomic[release _], (x + 1, 0) || (0, y), atomic[acquire _], (x, _)
+//! vs t: (x, 0), atomic[release _], (x + 1, 0) || (0, y), atomic[acquire _], (x+1, _)
+//! Both result in the sequence on thread x up to and including the atomic release as happening
+//! before the acquire.
+//!
 //! FIXME:
 //! currently we have our own local copy of the currently active thread index and names, this is due
 //! in part to the inability to access the current location of threads.active_thread inside the AllocExtra
@@ -499,7 +517,7 @@ 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
+    /// On success an atomic RMW operation is performed and on failure
     /// only an atomic read occurs.
     fn atomic_compare_exchange_scalar(
         &mut self,
@@ -1136,9 +1154,6 @@ impl GlobalState {
         // Now load the two clocks and configure the initial state.
         let (current, created) = vector_clocks.pick2_mut(current_index, created_index);
 
-        // Advance the current thread before the synchronized operation.
-        current.increment_clock(current_index);
-
         // Join the created with current, since the current threads
         // previous actions happen-before the created thread.
         created.join_with(current);
@@ -1167,14 +1182,12 @@ impl GlobalState {
             .as_ref()
             .expect("Joined with thread but thread has not terminated");
 
-        // Pre increment clocks before atomic operation.
-        current.increment_clock(current_index);
 
         // The join thread happens-before the current thread
         // so update the current vector clock.
         current.clock.join(join_clock);
 
-        // Post increment clocks after atomic operation.
+        // Increment clocks after atomic operation.
         current.increment_clock(current_index);
 
         // Check the number of active threads, if the value is 1
@@ -1277,8 +1290,7 @@ impl GlobalState {
         op: impl FnOnce(VectorIdx, RefMut<'_, ThreadClockSet>) -> InterpResult<'tcx>,
     ) -> InterpResult<'tcx> {
         if self.multi_threaded.get() {
-            let (index, mut clocks) = self.current_thread_state_mut();
-            clocks.increment_clock(index);
+            let (index, clocks) = self.current_thread_state_mut();
             op(index, clocks)?;
             let (_, mut clocks) = self.current_thread_state_mut();
             clocks.increment_clock(index);
@@ -1303,16 +1315,18 @@ impl GlobalState {
     /// `validate_lock_release` must happen before this.
     pub fn validate_lock_acquire(&self, lock: &VClock, thread: ThreadId) {
         let (index, mut clocks) = self.load_thread_state_mut(thread);
-        clocks.increment_clock(index);
         clocks.clock.join(&lock);
         clocks.increment_clock(index);
     }
 
     /// Release a lock handle, express that this happens-before
     /// any subsequent calls to `validate_lock_acquire`.
+    /// For normal locks this should be equivalent to `validate_lock_release_shared`
+    /// since an acquire operation should have occured before, however
+    /// for futex & cond-var operations this is not the case and this
+    /// operation must be used.
     pub fn validate_lock_release(&self, lock: &mut VClock, thread: ThreadId) {
         let (index, mut clocks) = self.load_thread_state_mut(thread);
-        clocks.increment_clock(index);
         lock.clone_from(&clocks.clock);
         clocks.increment_clock(index);
     }
@@ -1321,9 +1335,11 @@ impl GlobalState {
     /// any subsequent calls to `validate_lock_acquire` as well
     /// as any previous calls to this function after any
     /// `validate_lock_release` calls.
+    /// For normal locks this should be equivalent to `validate_lock_release`
+    /// this function only exists for joining over the set of concurrent readers
+    /// in a read-write lock and should not be used for anything else.
     pub fn validate_lock_release_shared(&self, lock: &mut VClock, thread: ThreadId) {
         let (index, mut clocks) = self.load_thread_state_mut(thread);
-        clocks.increment_clock(index);
         lock.join(&clocks.clock);
         clocks.increment_clock(index);
     }

src/shims/posix/sync.rs

@@ -62,9 +62,11 @@ fn mutex_get_kind<'mir, 'tcx: 'mir>(
     mutex_op: OpTy<'tcx, Tag>,
 ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     let offset = if ecx.pointer_size().bytes() == 8 { 16 } else { 12 };
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // mutex implementation, it may not need to be atomic.
     ecx.read_scalar_at_offset_atomic(
         mutex_op, offset, ecx.machine.layouts.i32,
-        AtomicReadOp::Acquire
+        AtomicReadOp::Relaxed
     )
 }
 
@@ -74,9 +76,11 @@ fn mutex_set_kind<'mir, 'tcx: 'mir>(
     kind: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     let offset = if ecx.pointer_size().bytes() == 8 { 16 } else { 12 };
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // mutex implementation, it may not need to be atomic.
     ecx.write_scalar_at_offset_atomic(
-        mutex_op, offset, kind, ecx.machine.layouts.i32, 
-        AtomicWriteOp::Release
+        mutex_op, offset, kind, ecx.machine.layouts.i32,
+        AtomicWriteOp::Relaxed
     )
 }
 
@@ -84,8 +88,11 @@ fn mutex_get_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     mutex_op: OpTy<'tcx, Tag>,
 ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // mutex implementation, it may not need to be atomic.
     ecx.read_scalar_at_offset_atomic(
-        mutex_op, 4, ecx.machine.layouts.u32, AtomicReadOp::Acquire
+        mutex_op, 4, ecx.machine.layouts.u32, 
+        AtomicReadOp::Relaxed
     )
 }
 
@@ -94,9 +101,11 @@ fn mutex_set_id<'mir, 'tcx: 'mir>(
     mutex_op: OpTy<'tcx, Tag>,
     id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // mutex implementation, it may not need to be atomic.
     ecx.write_scalar_at_offset_atomic(
         mutex_op, 4, id, ecx.machine.layouts.u32,
-        AtomicWriteOp::Release
+        AtomicWriteOp::Relaxed
     )
 }
 
@@ -126,10 +135,12 @@ fn mutex_get_or_create_id<'mir, 'tcx: 'mir>(
 fn rwlock_get_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     rwlock_op: OpTy<'tcx, Tag>,
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // rw-lock implementation, it may not need to be atomic.
 ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     ecx.read_scalar_at_offset_atomic(
         rwlock_op, 4, ecx.machine.layouts.u32,
-        AtomicReadOp::Acquire
+        AtomicReadOp::Relaxed
     )
 }
 
@@ -138,9 +149,11 @@ fn rwlock_set_id<'mir, 'tcx: 'mir>(
     rwlock_op: OpTy<'tcx, Tag>,
     id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // rw-lock implementation, it may not need to be atomic.
     ecx.write_scalar_at_offset_atomic(
         rwlock_op, 4, id, ecx.machine.layouts.u32,
-        AtomicWriteOp::Release
+        AtomicWriteOp::Relaxed
     )
 }
 
@@ -194,9 +207,11 @@ fn cond_get_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     cond_op: OpTy<'tcx, Tag>,
 ) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // cond-var implementation, it may not need to be atomic.
     ecx.read_scalar_at_offset_atomic(
         cond_op, 4, ecx.machine.layouts.u32,
-        AtomicReadOp::Acquire
+        AtomicReadOp::Relaxed
     )
 }
 
@@ -205,9 +220,11 @@ fn cond_set_id<'mir, 'tcx: 'mir>(
     cond_op: OpTy<'tcx, Tag>,
     id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
+    //FIXME: this has been made atomic to fix data-race reporting inside the internal
+    // cond-var implementation, it may not need to be atomic.
     ecx.write_scalar_at_offset_atomic(
         cond_op, 4, id, ecx.machine.layouts.u32,
-        AtomicWriteOp::Release
+        AtomicWriteOp::Relaxed
     )
 }
 

src/shims/posix/thread.rs

@@ -15,7 +15,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let this = self.eval_context_mut();
 
         this.tcx.sess.warn(
-            "thread support is experimental, no weak memory effects are currently emulated.",
+            "thread support is experimental and incomplete: weak memory effects are not emulated."
         );
 
         // Create the new thread

tests/compile-fail/data_race/dangling_thread_async_race.rs

@@ -29,9 +29,9 @@ fn main() {
     sleep(Duration::from_millis(100));
 
     // Spawn and immediately join a thread
-    //  to execute the join code-path
-    //  and ensure that data-race detection
-    //  remains enabled
+    // to execute the join code-path
+    // and ensure that data-race detection
+    // remains enabled nevertheless.
     spawn(|| ()).join().unwrap();
 
     let join2 = unsafe {

tests/compile-fail/data_race/dangling_thread_race.rs

@@ -29,9 +29,9 @@ fn main() {
     sleep(Duration::from_millis(100));
 
     // Spawn and immediately join a thread
-    //  to execute the join code-path
-    //  and ensure that data-race detection
-    //  remains enabled
+    // to execute the join code-path
+    // and ensure that data-race detection
+    // remains enabled nevertheless.
     spawn(|| ()).join().unwrap();
 
 

tests/compile-fail/data_race/enable_after_join_to_main.rs

@@ -9,7 +9,7 @@ unsafe impl<T> Send for EvilSend<T> {}
 unsafe impl<T> Sync for EvilSend<T> {}
 
 pub fn main() {
-    // Enable and the join with multiple threads
+    // Enable and then join with multiple threads.
     let t1 = spawn(|| ());
     let t2 = spawn(|| ());
     let t3 = spawn(|| ());
@@ -19,7 +19,7 @@ pub fn main() {
     t3.join().unwrap();
     t4.join().unwrap();
 
-    // Perform write-write data race detection
+    // Perform write-write data race detection.
     let mut a = 0u32;
     let b = &mut a as *mut u32;
     let c = EvilSend(b);

tests/compile-fail/data_race/relax_acquire_race.rs

@@ -16,6 +16,13 @@ pub fn main() {
     let b = &mut a as *mut u32;
     let c = EvilSend(b);
 
+    // Note: this is scheduler-dependent
+    // the operations need to occur in
+    // order:
+    //  1. store release : 1
+    //  2. load acquire : 1
+    //  3. store relaxed : 2
+    //  4. load acquire : 2
     unsafe {
         let j1 = spawn(move || {
             *c.0 = 1;

tests/compile-fail/data_race/release_seq_race.rs

@@ -18,6 +18,14 @@ pub fn main() {
     let b = &mut a as *mut u32;
     let c = EvilSend(b);
 
+    // Note: this is scheduler-dependent
+    // the operations need to occur in
+    // order, the sleep operations currently
+    // force the desired ordering:
+    //  1. store release : 1
+    //  2. store relaxed : 2
+    //  3. store relaxed : 3
+    //  4. load acquire : 3
     unsafe {
         let j1 = spawn(move || {
             *c.0 = 1;

tests/compile-fail/data_race/rmw_race.rs

@@ -16,6 +16,13 @@ pub fn main() {
     let b = &mut a as *mut u32;
     let c = EvilSend(b);
 
+    // Note: this is scheduler-dependent
+    // the operations need to occur in
+    // order:
+    //  1. store release : 1
+    //  2. RMW relaxed : 1 -> 2
+    //  3. store relaxed : 3
+    //  4. load acquire : 3
     unsafe {
         let j1 = spawn(move || {
             *c.0 = 1;

tests/run-pass/concurrency/data_race.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/concurrency/disable_data_race_detector.rs

@@ -19,7 +19,7 @@ pub fn main() {
         });
 
         let j2 = spawn(move || {
-            *c.0 = 64; //~ ERROR Data race
+            *c.0 = 64; //~ ERROR Data race (but not detected as the detector is disabled)
         });
 
         j1.join().unwrap();

tests/run-pass/concurrency/disable_data_race_detector.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/concurrency/linux-futex.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/concurrency/simple.stderr

@@ -1,4 +1,4 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 
 thread '<unnamed>' panicked at 'Hello!', $DIR/simple.rs:54:9
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/run-pass/concurrency/sync.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/concurrency/thread_locals.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/concurrency/tls_lib_drop.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/libc.stderr

@@ -1,2 +1,2 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 

tests/run-pass/panic/concurrent-panic.stderr

@@ -1,4 +1,4 @@
-warning: thread support is experimental, no weak memory effects are currently emulated.
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
 
 Thread 1 starting, will block on mutex
 Thread 1 reported it has started