Auto merge of #2054 - m-ou-se:futex-wait-bitset, r=RalfJung

Add support for FUTEX_{WAIT,WAKE}_BITSET

FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET are extensions of FUTEX_WAIT and FUTEX_WAKE that allow tagging each waiting thread with up to 32 'labels', and then only wake up threads that match certain labels. The non-bitset operations behave like their bitset was fully set (u32::MAX), meaning that they'll wait for anything, and wake up anything.

The only other difference is that FUTEX_WAIT_BITSET uses an absolute timeout instead of an relative timeout like FUTEX_WAIT.

Often, FUTEX_WAIT_BITSET is used not for its bitset functionality, but only for its absolute timeout functionality. It is then used with a bitset of u32::MAX.

~~This adds support for only that use case to Miri, as that's all `std` currently needs. Any other bitset is still unsupported.~~

Update: This adds full support for both these syscalls.

Author: bors

rust-version

@@ -1 +1 @@
-f262ca12aac76152c4b46cefcf8300f0249a5eb2
+306ba8357fb36212b7d30efb9eb9e41659ac1445

src/shims/posix/linux/sync.rs

@@ -36,7 +36,9 @@ pub fn futex<'tcx>(
 
     let futex_private = this.eval_libc_i32("FUTEX_PRIVATE_FLAG")?;
     let futex_wait = this.eval_libc_i32("FUTEX_WAIT")?;
+    let futex_wait_bitset = this.eval_libc_i32("FUTEX_WAIT_BITSET")?;
     let futex_wake = this.eval_libc_i32("FUTEX_WAKE")?;
+    let futex_wake_bitset = this.eval_libc_i32("FUTEX_WAKE_BITSET")?;
     let futex_realtime = this.eval_libc_i32("FUTEX_CLOCK_REALTIME")?;
 
     // FUTEX_PRIVATE enables an optimization that stops it from working across processes.
@@ -45,12 +47,37 @@ pub fn futex<'tcx>(
         // FUTEX_WAIT: (int *addr, int op = FUTEX_WAIT, int val, const timespec *timeout)
         // Blocks the thread if *addr still equals val. Wakes up when FUTEX_WAKE is called on the same address,
         // or *timeout expires. `timeout == null` for an infinite timeout.
-        op if op & !futex_realtime == futex_wait => {
-            if args.len() < 5 {
-                throw_ub_format!(
-                    "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT`: got {}, expected at least 5",
-                    args.len()
-                );
+        //
+        // FUTEX_WAIT_BITSET: (int *addr, int op = FUTEX_WAIT_BITSET, int val, const timespec *timeout, int *_ignored, unsigned int bitset)
+        // This is identical to FUTEX_WAIT, except:
+        //  - The timeout is absolute rather than relative.
+        //  - You can specify the bitset to selecting what WAKE operations to respond to.
+        op if op & !futex_realtime == futex_wait || op & !futex_realtime == futex_wait_bitset => {
+            let wait_bitset = op & !futex_realtime == futex_wait_bitset;
+
+            let bitset = if wait_bitset {
+                if args.len() != 7 {
+                    throw_ub_format!(
+                        "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT_BITSET`: got {}, expected 7",
+                        args.len()
+                    );
+                }
+                this.read_scalar(&args[6])?.to_u32()?
+            } else {
+                if args.len() < 5 {
+                    throw_ub_format!(
+                        "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT`: got {}, expected at least 5",
+                        args.len()
+                    );
+                }
+                u32::MAX
+            };
+
+            if bitset == 0 {
+                let einval = this.eval_libc("EINVAL")?;
+                this.set_last_error(einval)?;
+                this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
+                return Ok(());
             }
 
             // `deref_operand` but not actually dereferencing the ptr yet (it might be NULL!).
@@ -70,10 +97,20 @@ pub fn futex<'tcx>(
                         return Ok(());
                     }
                 };
-                Some(if op & futex_realtime != 0 {
-                    Time::RealTime(SystemTime::now().checked_add(duration).unwrap())
+                Some(if wait_bitset {
+                    // FUTEX_WAIT_BITSET uses an absolute timestamp.
+                    if op & futex_realtime != 0 {
+                        Time::RealTime(SystemTime::UNIX_EPOCH.checked_add(duration).unwrap())
+                    } else {
+                        Time::Monotonic(this.machine.time_anchor.checked_add(duration).unwrap())
+                    }
                 } else {
-                    Time::Monotonic(Instant::now().checked_add(duration).unwrap())
+                    // FUTEX_WAIT uses a relative timestamp.
+                    if op & futex_realtime != 0 {
+                        Time::RealTime(SystemTime::now().checked_add(duration).unwrap())
+                    } else {
+                        Time::Monotonic(Instant::now().checked_add(duration).unwrap())
+                    }
                 })
             };
             // Check the pointer for alignment and validity.
@@ -108,7 +145,7 @@ pub fn futex<'tcx>(
             if val == futex_val {
                 // The value still matches, so we block the trait make it wait for FUTEX_WAKE.
                 this.block_thread(thread);
-                this.futex_wait(addr_scalar.to_machine_usize(this)?, thread);
+                this.futex_wait(addr_scalar.to_machine_usize(this)?, thread, bitset);
                 // Succesfully waking up from FUTEX_WAIT always returns zero.
                 this.write_scalar(Scalar::from_machine_isize(0, this), dest)?;
                 // Register a timeout callback if a timeout was specified.
@@ -140,10 +177,29 @@ pub fn futex<'tcx>(
         // Wakes at most `val` threads waiting on the futex at `addr`.
         // Returns the amount of threads woken up.
         // Does not access the futex value at *addr.
-        op if op == futex_wake => {
+        // FUTEX_WAKE_BITSET: (int *addr, int op = FUTEX_WAKE, int val, const timespect *_unused, int *_unused, unsigned int bitset)
+        // Same as FUTEX_WAKE, but allows you to specify a bitset to select which threads to wake up.
+        op if op == futex_wake || op == futex_wake_bitset => {
+            let bitset = if op == futex_wake_bitset {
+                if args.len() != 7 {
+                    throw_ub_format!(
+                        "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAKE_BITSET`: got {}, expected 7",
+                        args.len()
+                    );
+                }
+                this.read_scalar(&args[6])?.to_u32()?
+            } else {
+                u32::MAX
+            };
+            if bitset == 0 {
+                let einval = this.eval_libc("EINVAL")?;
+                this.set_last_error(einval)?;
+                this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
+                return Ok(());
+            }
             let mut n = 0;
             for _ in 0..val {
-                if let Some(thread) = this.futex_wake(addr_scalar.to_machine_usize(this)?) {
+                if let Some(thread) = this.futex_wake(addr_scalar.to_machine_usize(this)?, bitset) {
                     this.unblock_thread(thread);
                     this.unregister_timeout_callback_if_exists(thread);
                     n += 1;

src/sync.rs

@@ -144,6 +144,8 @@ struct Futex {
 struct FutexWaiter {
     /// The thread that is waiting on this futex.
     thread: ThreadId,
+    /// The bitset used by FUTEX_*_BITSET, or u32::MAX for other operations.
+    bitset: u32,
 }
 
 /// The state of all synchronization variables.
@@ -486,15 +488,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.machine.threads.sync.condvars[id].waiters.retain(|waiter| waiter.thread != thread);
     }
 
-    fn futex_wait(&mut self, addr: u64, thread: ThreadId) {
+    fn futex_wait(&mut self, addr: u64, thread: ThreadId, bitset: u32) {
         let this = self.eval_context_mut();
         let futex = &mut this.machine.threads.sync.futexes.entry(addr).or_default();
         let waiters = &mut futex.waiters;
         assert!(waiters.iter().all(|waiter| waiter.thread != thread), "thread is already waiting");
-        waiters.push_back(FutexWaiter { thread });
+        waiters.push_back(FutexWaiter { thread, bitset });
     }
 
-    fn futex_wake(&mut self, addr: u64) -> Option<ThreadId> {
+    fn futex_wake(&mut self, addr: u64, bitset: u32) -> Option<ThreadId> {
         let this = self.eval_context_mut();
         let current_thread = this.get_active_thread();
         let futex = &mut this.machine.threads.sync.futexes.get_mut(&addr)?;
@@ -504,13 +506,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         if let Some(data_race) = data_race {
             data_race.validate_lock_release(&mut futex.data_race, current_thread);
         }
-        let res = futex.waiters.pop_front().map(|waiter| {
+
+        // Wake up the first thread in the queue that matches any of the bits in the bitset.
+        futex.waiters.iter().position(|w| w.bitset & bitset != 0).map(|i| {
+            let waiter = futex.waiters.remove(i).unwrap();
             if let Some(data_race) = data_race {
                 data_race.validate_lock_acquire(&futex.data_race, waiter.thread);
             }
             waiter.thread
-        });
-        res
+        })
     }
 
     fn futex_remove_waiter(&mut self, addr: u64, thread: ThreadId) {

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

@@ -7,6 +7,7 @@
 #![feature(rustc_private)]
 extern crate libc;
 
+use std::mem::MaybeUninit;
 use std::ptr;
 use std::thread;
 use std::time::{Duration, Instant};
@@ -93,6 +94,42 @@ fn wait_timeout() {
     assert!((200..1000).contains(&start.elapsed().as_millis()));
 }
 
+fn wait_absolute_timeout() {
+    let start = Instant::now();
+
+    // Get the current monotonic timestamp as timespec.
+    let mut timeout = unsafe {
+        let mut now: MaybeUninit<libc::timespec> = MaybeUninit::uninit();
+        assert_eq!(libc::clock_gettime(libc::CLOCK_MONOTONIC, now.as_mut_ptr()), 0);
+        now.assume_init()
+    };
+
+    // Add 200ms.
+    timeout.tv_nsec += 200_000_000;
+    if timeout.tv_nsec > 1_000_000_000 {
+        timeout.tv_nsec -= 1_000_000_000;
+        timeout.tv_sec += 1;
+    }
+
+    let futex: i32 = 123;
+
+    // Wait for 200ms from now, with nobody waking us up early.
+    unsafe {
+        assert_eq!(libc::syscall(
+            libc::SYS_futex,
+            &futex as *const i32,
+            libc::FUTEX_WAIT_BITSET,
+            123,
+            &timeout,
+            0,
+            u32::MAX,
+        ), -1);
+        assert_eq!(*libc::__errno_location(), libc::ETIMEDOUT);
+    }
+
+    assert!((200..1000).contains(&start.elapsed().as_millis()));
+}
+
 fn wait_wake() {
     let start = Instant::now();
 
@@ -123,10 +160,59 @@ fn wait_wake() {
     assert!((200..1000).contains(&start.elapsed().as_millis()));
 }
 
+fn wait_wake_bitset() {
+    let start = Instant::now();
+
+    static FUTEX: i32 = 0;
+
+    thread::spawn(move || {
+        thread::sleep(Duration::from_millis(200));
+        unsafe {
+            assert_eq!(libc::syscall(
+                libc::SYS_futex,
+                &FUTEX as *const i32,
+                libc::FUTEX_WAKE_BITSET,
+                10, // Wake up at most 10 threads.
+                0,
+                0,
+                0b1001, // bitset
+            ), 0); // Didn't match any thread.
+        }
+        thread::sleep(Duration::from_millis(200));
+        unsafe {
+            assert_eq!(libc::syscall(
+                libc::SYS_futex,
+                &FUTEX as *const i32,
+                libc::FUTEX_WAKE_BITSET,
+                10, // Wake up at most 10 threads.
+                0,
+                0,
+                0b0110, // bitset
+            ), 1); // Woken up one thread.
+        }
+    });
+
+    unsafe {
+        assert_eq!(libc::syscall(
+            libc::SYS_futex,
+            &FUTEX as *const i32,
+            libc::FUTEX_WAIT_BITSET,
+            0,
+            ptr::null::<libc::timespec>(),
+            0,
+            0b0100, // bitset
+        ), 0);
+    }
+
+    assert!((400..1000).contains(&start.elapsed().as_millis()));
+}
+
 fn main() {
     wake_nobody();
     wake_dangling();
     wait_wrong_val();
     wait_timeout();
+    wait_absolute_timeout();
     wait_wake();
+    wait_wake_bitset();
 }