Add vector clock to epoll ready lists

src/shims/unix/linux/epoll.rs

@@ -4,6 +4,7 @@ use std::io;
 use std::rc::{Rc, Weak};
 use std::time::Duration;
 
+use crate::concurrency::VClock;
 use crate::shims::unix::fd::{FdId, FileDescriptionRef, WeakFileDescriptionRef};
 use crate::shims::unix::*;
 use crate::*;
@@ -19,7 +20,7 @@ struct Epoll {
     /// and file descriptor value.
     // This is an Rc because EpollInterest need to hold a reference to update
     // it.
-    ready_list: Rc<RefCell<BTreeMap<(FdId, i32), EpollEventInstance>>>,
+    ready_list: Rc<ReadyList>,
     /// A list of thread ids blocked on this epoll instance.
     thread_id: RefCell<Vec<ThreadId>>,
 }
@@ -63,7 +64,7 @@ pub struct EpollEventInterest {
     /// <https://man7.org/linux/man-pages/man3/epoll_event.3type.html>
     data: u64,
     /// Ready list of the epoll instance under which this EpollEventInterest is registered.
-    ready_list: Rc<RefCell<BTreeMap<(FdId, i32), EpollEventInstance>>>,
+    ready_list: Rc<ReadyList>,
     /// The epoll file description that this EpollEventInterest is registered under.
     weak_epfd: WeakFileDescriptionRef,
 }
@@ -88,6 +89,12 @@ pub struct EpollReadyEvents {
     pub epollerr: bool,
 }
 
+#[derive(Debug, Default)]
+struct ReadyList {
+    mapping: RefCell<BTreeMap<(FdId, i32), EpollEventInstance>>,
+    clock: RefCell<VClock>,
+}
+
 impl EpollReadyEvents {
     pub fn new() -> Self {
         EpollReadyEvents {
@@ -127,7 +134,7 @@ impl EpollReadyEvents {
 }
 
 impl Epoll {
-    fn get_ready_list(&self) -> Rc<RefCell<BTreeMap<(FdId, i32), EpollEventInstance>>> {
+    fn get_ready_list(&self) -> Rc<ReadyList> {
         Rc::clone(&self.ready_list)
     }
 }
@@ -207,9 +214,6 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             );
         }
 
-        let mut epoll_instance = Epoll::default();
-        epoll_instance.ready_list = Rc::new(RefCell::new(BTreeMap::new()));
-
         let fd = this.machine.fds.insert_new(Epoll::default());
         Ok(Scalar::from_i32(fd))
     }
@@ -377,7 +381,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             drop(epoll_interest);
 
             // Remove related epoll_interest from ready list.
-            ready_list.borrow_mut().remove(&epoll_key);
+            ready_list.mapping.borrow_mut().remove(&epoll_key);
 
             // Remove dangling EpollEventInterest from its global table.
             // .unwrap() below should succeed because the file description id must have registered
@@ -471,8 +475,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             let epoll_file_description = epfd
                 .downcast::<Epoll>()
                 .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
-            let binding = epoll_file_description.get_ready_list();
-            ready_list_empty = binding.borrow_mut().is_empty();
+            ready_list_empty = epoll_file_description.ready_list.mapping.borrow().is_empty();
             thread_ids = epoll_file_description.thread_id.borrow_mut();
         }
         if timeout == 0 || !ready_list_empty {
@@ -561,9 +564,15 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                         // holds a strong ref to epoll_interest.
                         let epfd = epoll_interest.borrow().weak_epfd.upgrade().unwrap();
                         // FIXME: We can randomly pick a thread to unblock.
-                        if let Some(thread_id) =
-                            epfd.downcast::<Epoll>().unwrap().thread_id.borrow_mut().pop()
-                        {
+
+                        let epoll = epfd.downcast::<Epoll>().unwrap();
+
+                        // Synchronize running thread to the epoll ready list.
+                        if let Some(clock) = &this.release_clock() {
+                            epoll.ready_list.clock.borrow_mut().join(clock);
+                        }
+
+                        if let Some(thread_id) = epoll.thread_id.borrow_mut().pop() {
                             waiter.push(thread_id);
                         };
                     }
@@ -617,7 +626,7 @@ fn check_and_update_one_event_interest<'tcx>(
     // insert an epoll_return to the ready list.
     if flags != 0 {
         let epoll_key = (id, epoll_event_interest.fd_num);
-        let ready_list = &mut epoll_event_interest.ready_list.borrow_mut();
+        let ready_list = &mut epoll_event_interest.ready_list.mapping.borrow_mut();
         let event_instance = EpollEventInstance::new(flags, epoll_event_interest.data);
         // Triggers the notification by inserting it to the ready list.
         ready_list.insert(epoll_key, event_instance);
@@ -644,7 +653,11 @@ fn blocking_epoll_callback<'tcx>(
         .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
 
     let ready_list = epoll_file_description.get_ready_list();
-    let mut ready_list = ready_list.borrow_mut();
+
+    // Synchronize waking thread from the epoll ready list.
+    ecx.acquire_clock(&ready_list.clock.borrow());
+
+    let mut ready_list = ready_list.mapping.borrow_mut();
     let mut num_of_events: i32 = 0;
     let mut array_iter = ecx.project_array_fields(events)?;
 

tests/pass-dep/libc/libc-epoll-blocking.rs

@@ -1,5 +1,5 @@
 //@only-target: linux
-// test_epoll_block_then_unblock depends on a deterministic schedule.
+// test_epoll_block_then_unblock and test_epoll_race depend on a deterministic schedule.
 //@compile-flags: -Zmiri-preemption-rate=0
 
 use std::convert::TryInto;
@@ -12,6 +12,7 @@ fn main() {
     test_epoll_block_without_notification();
     test_epoll_block_then_unblock();
     test_notification_after_timeout();
+    test_epoll_race();
 }
 
 // Using `as` cast since `EPOLLET` wraps around
@@ -137,3 +138,41 @@ fn test_notification_after_timeout() {
     let expected_value = fds[0] as u64;
     check_epoll_wait::<1>(epfd, &[(expected_event, expected_value)], 10);
 }
+
+// This test shows a data_race before epoll had vector clocks added.
+fn test_epoll_race() {
+    // Create an epoll instance.
+    let epfd = unsafe { libc::epoll_create1(0) };
+    assert_ne!(epfd, -1);
+
+    // Create an eventfd instance.
+    let flags = libc::EFD_NONBLOCK | libc::EFD_CLOEXEC;
+    let fd = unsafe { libc::eventfd(0, flags) };
+
+    // Register eventfd with the epoll instance.
+    let mut ev = libc::epoll_event { events: EPOLL_IN_OUT_ET, u64: fd as u64 };
+    let res = unsafe { libc::epoll_ctl(epfd, libc::EPOLL_CTL_ADD, fd, &mut ev) };
+    assert_eq!(res, 0);
+
+    static mut VAL: u8 = 0;
+    let thread1 = thread::spawn(move || {
+        // Write to the static mut variable.
+        unsafe { VAL = 1 };
+        // Write to the eventfd instance.
+        let sized_8_data: [u8; 8] = 1_u64.to_ne_bytes();
+        let res = unsafe { libc::write(fd, sized_8_data.as_ptr() as *const libc::c_void, 8) };
+        // read returns number of bytes that have been read, which is always 8.
+        assert_eq!(res, 8);
+    });
+    thread::yield_now();
+    // epoll_wait for the event to happen.
+    let expected_event = u32::try_from(libc::EPOLLIN | libc::EPOLLOUT).unwrap();
+    let expected_value = u64::try_from(fd).unwrap();
+    check_epoll_wait::<8>(epfd, &[(expected_event, expected_value)], -1);
+    // Read from the static mut variable.
+    #[allow(static_mut_refs)]
+    unsafe {
+        assert_eq!(VAL, 1)
+    };
+    thread1.join().unwrap();
+}