Auto merge of rust-lang#3495 - RalfJung:data-race-clocks, r=RalfJung

data_race: make the release/acquire API more clear

src/tools/miri/src/alloc_addresses/mod.rs

@@ -13,7 +13,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_span::Span;
 use rustc_target::abi::{Align, HasDataLayout, Size};
 
-use crate::*;
+use crate::{concurrency::VClock, *};
 
 use self::reuse_pool::ReusePool;
 
@@ -171,8 +171,10 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                     memory_kind,
                     ecx.get_active_thread(),
                 ) {
-                    if let Some(data_race) = &ecx.machine.data_race {
-                        data_race.validate_lock_acquire(&clock, ecx.get_active_thread());
+                    if let Some(clock) = clock
+                        && let Some(data_race) = &ecx.machine.data_race
+                    {
+                        data_race.acquire_clock(&clock, ecx.get_active_thread());
                     }
                     reuse_addr
                 } else {
@@ -369,15 +371,13 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
         // Also remember this address for future reuse.
         let thread = self.threads.get_active_thread_id();
         global_state.reuse.add_addr(rng, addr, size, align, kind, thread, || {
-            let mut clock = concurrency::VClock::default();
             if let Some(data_race) = &self.data_race {
-                data_race.validate_lock_release(
-                    &mut clock,
-                    thread,
-                    self.threads.active_thread_ref().current_span(),
-                );
+                data_race
+                    .release_clock(thread, self.threads.active_thread_ref().current_span())
+                    .clone()
+            } else {
+                VClock::default()
             }
-            clock
         })
     }
 }

src/tools/miri/src/alloc_addresses/reuse_pool.rs

@@ -78,14 +78,15 @@ impl ReusePool {
         subpool.insert(pos, (addr, size, thread, clock));
     }
 
+    /// Returns the address to use and optionally a clock we have to synchronize with.
     pub fn take_addr(
         &mut self,
         rng: &mut impl Rng,
         size: Size,
         align: Align,
         kind: MemoryKind,
         thread: ThreadId,
-    ) -> Option<(u64, VClock)> {
+    ) -> Option<(u64, Option<VClock>)> {
         // Determine whether we'll even attempt a reuse. As above, we don't do reuse for stack addresses.
         if kind == MemoryKind::Stack || !rng.gen_bool(self.address_reuse_rate) {
             return None;
@@ -122,6 +123,7 @@ impl ReusePool {
         let (chosen_addr, chosen_size, chosen_thread, clock) = subpool.remove(idx);
         debug_assert!(chosen_size >= size && chosen_addr % align.bytes() == 0);
         debug_assert!(cross_thread_reuse || chosen_thread == thread);
-        Some((chosen_addr, clock))
+        // No synchronization needed if we reused from the current thread.
+        Some((chosen_addr, if chosen_thread == thread { None } else { Some(clock) }))
     }
 }

src/tools/miri/src/concurrency/data_race.rs

@@ -547,9 +547,9 @@ impl MemoryCellClocks {
     ) -> Result<(), DataRace> {
         trace!("Unsynchronized read with vectors: {:#?} :: {:#?}", self, thread_clocks);
         if !current_span.is_dummy() {
-            thread_clocks.clock[index].span = current_span;
+            thread_clocks.clock.index_mut(index).span = current_span;
         }
-        thread_clocks.clock[index].set_read_type(read_type);
+        thread_clocks.clock.index_mut(index).set_read_type(read_type);
         if self.write_was_before(&thread_clocks.clock) {
             let race_free = if let Some(atomic) = self.atomic() {
                 // We must be ordered-after all atomic accesses, reads and writes.
@@ -577,7 +577,7 @@ impl MemoryCellClocks {
     ) -> Result<(), DataRace> {
         trace!("Unsynchronized write with vectors: {:#?} :: {:#?}", self, thread_clocks);
         if !current_span.is_dummy() {
-            thread_clocks.clock[index].span = current_span;
+            thread_clocks.clock.index_mut(index).span = current_span;
         }
         if self.write_was_before(&thread_clocks.clock) && self.read <= thread_clocks.clock {
             let race_free = if let Some(atomic) = self.atomic() {
@@ -1701,49 +1701,34 @@ impl GlobalState {
         format!("thread `{thread_name}`")
     }
 
-    /// Acquire a lock, express that the previous call of
-    /// `validate_lock_release` must happen before this.
+    /// Acquire the given clock into the given thread, establishing synchronization with
+    /// the moment when that clock snapshot was taken via `release_clock`.
     /// As this is an acquire operation, the thread timestamp is not
     /// incremented.
-    pub fn validate_lock_acquire(&self, lock: &VClock, thread: ThreadId) {
-        let (_, mut clocks) = self.load_thread_state_mut(thread);
+    pub fn acquire_clock(&self, lock: &VClock, thread: ThreadId) {
+        let (_, mut clocks) = self.thread_state_mut(thread);
         clocks.clock.join(lock);
     }
 
-    /// 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 occurred before, however
-    /// for futex & condvar operations this is not the case and this
-    /// operation must be used.
-    pub fn validate_lock_release(&self, lock: &mut VClock, thread: ThreadId, current_span: Span) {
-        let (index, mut clocks) = self.load_thread_state_mut(thread);
-        lock.clone_from(&clocks.clock);
-        clocks.increment_clock(index, current_span);
-    }
-
-    /// Release a lock handle, express that this happens-before
-    /// 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,
-        current_span: Span,
-    ) {
-        let (index, mut clocks) = self.load_thread_state_mut(thread);
-        lock.join(&clocks.clock);
+    /// Returns the `release` clock of the given thread.
+    /// Other threads can acquire this clock in the future to establish synchronization
+    /// with this program point.
+    pub fn release_clock(&self, thread: ThreadId, current_span: Span) -> Ref<'_, VClock> {
+        // We increment the clock each time this happens, to ensure no two releases
+        // can be confused with each other.
+        let (index, mut clocks) = self.thread_state_mut(thread);
         clocks.increment_clock(index, current_span);
+        drop(clocks);
+        // To return a read-only view, we need to release the RefCell
+        // and borrow it again.
+        let (_index, clocks) = self.thread_state(thread);
+        Ref::map(clocks, |c| &c.clock)
     }
 
     /// Load the vector index used by the given thread as well as the set of vector clocks
     /// used by the thread.
     #[inline]
-    fn load_thread_state_mut(&self, thread: ThreadId) -> (VectorIdx, RefMut<'_, ThreadClockSet>) {
+    fn thread_state_mut(&self, thread: ThreadId) -> (VectorIdx, RefMut<'_, ThreadClockSet>) {
         let index = self.thread_info.borrow()[thread]
             .vector_index
             .expect("Loading thread state for thread with no assigned vector");
@@ -1752,17 +1737,26 @@ impl GlobalState {
         (index, clocks)
     }
 
+    /// Load the vector index used by the given thread as well as the set of vector clocks
+    /// used by the thread.
+    #[inline]
+    fn thread_state(&self, thread: ThreadId) -> (VectorIdx, Ref<'_, ThreadClockSet>) {
+        let index = self.thread_info.borrow()[thread]
+            .vector_index
+            .expect("Loading thread state for thread with no assigned vector");
+        let ref_vector = self.vector_clocks.borrow();
+        let clocks = Ref::map(ref_vector, |vec| &vec[index]);
+        (index, clocks)
+    }
+
     /// Load the current vector clock in use and the current set of thread clocks
     /// in use for the vector.
     #[inline]
     pub(super) fn current_thread_state(
         &self,
         thread_mgr: &ThreadManager<'_, '_>,
     ) -> (VectorIdx, Ref<'_, ThreadClockSet>) {
-        let index = self.current_index(thread_mgr);
-        let ref_vector = self.vector_clocks.borrow();
-        let clocks = Ref::map(ref_vector, |vec| &vec[index]);
-        (index, clocks)
+        self.thread_state(thread_mgr.get_active_thread_id())
     }
 
     /// Load the current vector clock in use and the current set of thread clocks
@@ -1772,10 +1766,7 @@ impl GlobalState {
         &self,
         thread_mgr: &ThreadManager<'_, '_>,
     ) -> (VectorIdx, RefMut<'_, ThreadClockSet>) {
-        let index = self.current_index(thread_mgr);
-        let ref_vector = self.vector_clocks.borrow_mut();
-        let clocks = RefMut::map(ref_vector, |vec| &mut vec[index]);
-        (index, clocks)
+        self.thread_state_mut(thread_mgr.get_active_thread_id())
     }
 
     /// Return the current thread, should be the same

src/tools/miri/src/concurrency/init_once.rs

@@ -41,7 +41,7 @@ pub enum InitOnceStatus {
 pub(super) struct InitOnce<'mir, 'tcx> {
     status: InitOnceStatus,
     waiters: VecDeque<InitOnceWaiter<'mir, 'tcx>>,
-    data_race: VClock,
+    clock: VClock,
 }
 
 impl<'mir, 'tcx> VisitProvenance for InitOnce<'mir, 'tcx> {
@@ -61,10 +61,8 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let current_thread = this.get_active_thread();
 
         if let Some(data_race) = &this.machine.data_race {
-            data_race.validate_lock_acquire(
-                &this.machine.threads.sync.init_onces[id].data_race,
-                current_thread,
-            );
+            data_race
+                .acquire_clock(&this.machine.threads.sync.init_onces[id].clock, current_thread);
         }
     }
 
@@ -176,7 +174,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
         // Each complete happens-before the end of the wait
         if let Some(data_race) = &this.machine.data_race {
-            data_race.validate_lock_release(&mut init_once.data_race, current_thread, current_span);
+            init_once.clock.clone_from(&data_race.release_clock(current_thread, current_span));
         }
 
         // Wake up everyone.
@@ -202,7 +200,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
         // Each complete happens-before the end of the wait
         if let Some(data_race) = &this.machine.data_race {
-            data_race.validate_lock_release(&mut init_once.data_race, current_thread, current_span);
+            init_once.clock.clone_from(&data_race.release_clock(current_thread, current_span));
         }
 
         // Wake up one waiting thread, so they can go ahead and try to init this.