fix behavior of release_clock()

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

@@ -453,7 +453,7 @@ impl<'tcx> MiriMachine<'tcx> {
         let thread = self.threads.active_thread();
         global_state.reuse.add_addr(rng, addr, size, align, kind, thread, || {
             if let Some(data_race) = &self.data_race {
-                data_race.release_clock(&self.threads).clone()
+                data_race.release_clock(&self.threads, |clock| clock.clone())
             } else {
                 VClock::default()
             }

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

@@ -828,15 +828,14 @@ pub trait EvalContextExt<'tcx>: MiriInterpCxExt<'tcx> {
         }
     }
 
-    /// Returns the `release` clock of the current thread.
+    /// Calls the callback with the "release" clock of the current thread.
     /// Other threads can acquire this clock in the future to establish synchronization
     /// with this program point.
-    fn release_clock<'a>(&'a self) -> Option<Ref<'a, VClock>>
-    where
-        'tcx: 'a,
-    {
+    ///
+    /// The closure will only be invoked if data race handling is on.
+    fn release_clock<R>(&self, callback: impl FnOnce(&VClock) -> R) -> Option<R> {
         let this = self.eval_context_ref();
-        Some(this.machine.data_race.as_ref()?.release_clock(&this.machine.threads))
+        Some(this.machine.data_race.as_ref()?.release_clock(&this.machine.threads, callback))
     }
 
     /// Acquire the given clock into the current thread, establishing synchronization with
@@ -1728,7 +1727,7 @@ impl GlobalState {
         let current_index = self.active_thread_index(thread_mgr);
 
         // Store the terminaion clock.
-        let terminaion_clock = self.release_clock(thread_mgr).clone();
+        let terminaion_clock = self.release_clock(thread_mgr, |clock| clock.clone());
         self.thread_info.get_mut()[current_thread].termination_vector_clock =
             Some(terminaion_clock);
 
@@ -1778,21 +1777,23 @@ impl GlobalState {
         clocks.clock.join(clock);
     }
 
-    /// Returns the `release` clock of the current thread.
+    /// Calls the given closure with the "release" clock of the current thread.
     /// Other threads can acquire this clock in the future to establish synchronization
     /// with this program point.
-    pub fn release_clock<'tcx>(&self, threads: &ThreadManager<'tcx>) -> Ref<'_, VClock> {
+    pub fn release_clock<'tcx, R>(
+        &self,
+        threads: &ThreadManager<'tcx>,
+        callback: impl FnOnce(&VClock) -> R,
+    ) -> R {
         let thread = threads.active_thread();
         let span = threads.active_thread_ref().current_span();
-        // 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);
+        let r = callback(&clocks.clock);
+        // Increment the clock, so that all following events cannot be confused with anything that
+        // occurred before the release. Crucially, the callback is invoked on the *old* clock!
         clocks.increment_clock(index, 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)
+
+        r
     }
 
     fn thread_index(&self, thread: ThreadId) -> VectorIdx {

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

@@ -93,7 +93,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Each complete happens-before the end of the wait
         if let Some(data_race) = &this.machine.data_race {
-            init_once.clock.clone_from(&data_race.release_clock(&this.machine.threads));
+            data_race
+                .release_clock(&this.machine.threads, |clock| init_once.clock.clone_from(clock));
         }
 
         // Wake up everyone.
@@ -119,7 +120,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Each complete happens-before the end of the wait
         if let Some(data_race) = &this.machine.data_race {
-            init_once.clock.clone_from(&data_race.release_clock(&this.machine.threads));
+            data_race
+                .release_clock(&this.machine.threads, |clock| init_once.clock.clone_from(clock));
         }
 
         // Wake up one waiting thread, so they can go ahead and try to init this.

src/tools/miri/src/concurrency/sync.rs

@@ -444,7 +444,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 // The mutex is completely unlocked. Try transferring ownership
                 // to another thread.
                 if let Some(data_race) = &this.machine.data_race {
-                    mutex.clock.clone_from(&data_race.release_clock(&this.machine.threads));
+                    data_race.release_clock(&this.machine.threads, |clock| {
+                        mutex.clock.clone_from(clock)
+                    });
                 }
                 if let Some(thread) = this.machine.sync.mutexes[id].queue.pop_front() {
                     this.unblock_thread(thread, BlockReason::Mutex(id))?;
@@ -553,7 +555,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         }
         if let Some(data_race) = &this.machine.data_race {
             // Add this to the shared-release clock of all concurrent readers.
-            rwlock.clock_current_readers.join(&data_race.release_clock(&this.machine.threads));
+            data_race.release_clock(&this.machine.threads, |clock| {
+                rwlock.clock_current_readers.join(clock)
+            });
         }
 
         // The thread was a reader. If the lock is not held any more, give it to a writer.
@@ -632,7 +636,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             trace!("rwlock_writer_unlock: {:?} unlocked by {:?}", id, thread);
             // Record release clock for next lock holder.
             if let Some(data_race) = &this.machine.data_race {
-                rwlock.clock_unlocked.clone_from(&*data_race.release_clock(&this.machine.threads));
+                data_race.release_clock(&this.machine.threads, |clock| {
+                    rwlock.clock_unlocked.clone_from(clock)
+                });
             }
             // The thread was a writer.
             //
@@ -764,7 +770,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Each condvar signal happens-before the end of the condvar wake
         if let Some(data_race) = data_race {
-            condvar.clock.clone_from(&*data_race.release_clock(&this.machine.threads));
+            data_race.release_clock(&this.machine.threads, |clock| condvar.clock.clone_from(clock));
         }
         let Some(waiter) = condvar.waiters.pop_front() else {
             return interp_ok(false);
@@ -837,7 +843,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Each futex-wake happens-before the end of the futex wait
         if let Some(data_race) = data_race {
-            futex.clock.clone_from(&*data_race.release_clock(&this.machine.threads));
+            data_race.release_clock(&this.machine.threads, |clock| futex.clock.clone_from(clock));
         }
 
         // Wake up the first thread in the queue that matches any of the bits in the bitset.

src/tools/miri/src/shims/unix/linux/epoll.rs

@@ -568,9 +568,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                         let epoll = epfd.downcast::<Epoll>().unwrap();
 
                         // Synchronize running thread to the epoll ready list.
-                        if let Some(clock) = &this.release_clock() {
+                        this.release_clock(|clock| {
                             epoll.ready_list.clock.borrow_mut().join(clock);
-                        }
+                        });
 
                         if let Some(thread_id) = epoll.thread_id.borrow_mut().pop() {
                             waiter.push(thread_id);

src/tools/miri/src/shims/unix/linux/eventfd.rs

@@ -140,9 +140,9 @@ impl FileDescription for Event {
         match self.counter.get().checked_add(num) {
             Some(new_count @ 0..=MAX_COUNTER) => {
                 // Future `read` calls will synchronize with this write, so update the FD clock.
-                if let Some(clock) = &ecx.release_clock() {
+                ecx.release_clock(|clock| {
                     self.clock.borrow_mut().join(clock);
-                }
+                });
                 self.counter.set(new_count);
             }
             None | Some(u64::MAX) =>

src/tools/miri/src/shims/unix/unnamed_socket.rs

@@ -234,9 +234,9 @@ impl FileDescription for AnonSocket {
             }
         }
         // Remember this clock so `read` can synchronize with us.
-        if let Some(clock) = &ecx.release_clock() {
+        ecx.release_clock(|clock| {
             writebuf.clock.join(clock);
-        }
+        });
         // Do full write / partial write based on the space available.
         let actual_write_size = len.min(available_space);
         let bytes = ecx.read_bytes_ptr_strip_provenance(ptr, Size::from_bytes(len))?;

src/tools/miri/tests/fail-dep/libc/socketpair-data-race.rs

@@ -0,0 +1,31 @@
+//! This is a regression test for <https://github.com/rust-lang/miri/issues/3947>: we had some
+//! faulty logic around `release_clock` that led to this code not reporting a data race.
+//@ignore-target: windows # no libc socketpair on Windows
+//@compile-flags: -Zmiri-preemption-rate=0
+use std::thread;
+
+fn main() {
+    static mut VAL: u8 = 0;
+    let mut fds = [-1, -1];
+    let res = unsafe { libc::socketpair(libc::AF_UNIX, libc::SOCK_STREAM, 0, fds.as_mut_ptr()) };
+    assert_eq!(res, 0);
+    let thread1 = thread::spawn(move || {
+        let data = "a".as_bytes().as_ptr();
+        let res = unsafe { libc::write(fds[0], data as *const libc::c_void, 1) };
+        assert_eq!(res, 1);
+        // The write to VAL is *after* the write to the socket, so there's no proper synchronization.
+        unsafe { VAL = 1 };
+    });
+    thread::yield_now();
+
+    let mut buf: [u8; 1] = [0; 1];
+    let res: i32 = unsafe {
+        libc::read(fds[1], buf.as_mut_ptr().cast(), buf.len() as libc::size_t).try_into().unwrap()
+    };
+    assert_eq!(res, 1);
+    assert_eq!(buf, "a".as_bytes());
+
+    unsafe { assert_eq!({ VAL }, 1) }; //~ERROR: Data race
+
+    thread1.join().unwrap();
+}

src/tools/miri/tests/fail-dep/libc/socketpair-data-race.stderr

@@ -0,0 +1,20 @@
+error: Undefined Behavior: Data race detected between (1) non-atomic write on thread `unnamed-ID` and (2) non-atomic read on thread `main` at ALLOC. (2) just happened here
+  --> tests/fail-dep/libc/socketpair-data-race.rs:LL:CC
+   |
+LL |     unsafe { assert_eq!({ VAL }, 1) };
+   |                           ^^^ Data race detected between (1) non-atomic write on thread `unnamed-ID` and (2) non-atomic read on thread `main` at ALLOC. (2) just happened here
+   |
+help: and (1) occurred earlier here
+  --> tests/fail-dep/libc/socketpair-data-race.rs:LL:CC
+   |
+LL |         unsafe { VAL = 1 };
+   |                  ^^^^^^^
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at tests/fail-dep/libc/socketpair-data-race.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+