Move boxing and mutex checking logic of condvar into sys_common.

Author: m-ou-se

library/std/src/sync/condvar.rs

@@ -2,10 +2,8 @@
 mod tests;
 
 use crate::fmt;
-use crate::sync::atomic::{AtomicUsize, Ordering};
 use crate::sync::{mutex, MutexGuard, PoisonError};
 use crate::sys_common::condvar as sys;
-use crate::sys_common::mutex as sys_mutex;
 use crate::sys_common::poison::{self, LockResult};
 use crate::time::{Duration, Instant};
 
@@ -109,8 +107,7 @@ impl WaitTimeoutResult {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Condvar {
-    inner: Box<sys::Condvar>,
-    mutex: AtomicUsize,
+    inner: sys::Condvar,
 }
 
 impl Condvar {
@@ -126,11 +123,7 @@ impl Condvar {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> Condvar {
-        let mut c = Condvar { inner: box sys::Condvar::new(), mutex: AtomicUsize::new(0) };
-        unsafe {
-            c.inner.init();
-        }
-        c
+        Condvar { inner: sys::Condvar::new() }
     }
 
     /// Blocks the current thread until this condition variable receives a
@@ -192,7 +185,6 @@ impl Condvar {
     pub fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> LockResult<MutexGuard<'a, T>> {
         let poisoned = unsafe {
             let lock = mutex::guard_lock(&guard);
-            self.verify(lock);
             self.inner.wait(lock);
             mutex::guard_poison(&guard).get()
         };
@@ -389,7 +381,6 @@ impl Condvar {
     ) -> LockResult<(MutexGuard<'a, T>, WaitTimeoutResult)> {
         let (poisoned, result) = unsafe {
             let lock = mutex::guard_lock(&guard);
-            self.verify(lock);
             let success = self.inner.wait_timeout(lock, dur);
             (mutex::guard_poison(&guard).get(), WaitTimeoutResult(!success))
         };
@@ -510,7 +501,7 @@ impl Condvar {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn notify_one(&self) {
-        unsafe { self.inner.notify_one() }
+        self.inner.notify_one()
     }
 
     /// Wakes up all blocked threads on this condvar.
@@ -550,27 +541,7 @@ impl Condvar {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn notify_all(&self) {
-        unsafe { self.inner.notify_all() }
-    }
-
-    fn verify(&self, mutex: &sys_mutex::MovableMutex) {
-        let addr = mutex.raw() as *const _ as usize;
-        match self.mutex.compare_and_swap(0, addr, Ordering::SeqCst) {
-            // If we got out 0, then we have successfully bound the mutex to
-            // this cvar.
-            0 => {}
-
-            // If we get out a value that's the same as `addr`, then someone
-            // already beat us to the punch.
-            n if n == addr => {}
-
-            // Anything else and we're using more than one mutex on this cvar,
-            // which is currently disallowed.
-            _ => panic!(
-                "attempted to use a condition variable with two \
-                         mutexes"
-            ),
-        }
+        self.inner.notify_all()
     }
 }
 
@@ -588,10 +559,3 @@ impl Default for Condvar {
         Condvar::new()
     }
 }
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Drop for Condvar {
-    fn drop(&mut self) {
-        unsafe { self.inner.destroy() }
-    }
-}

library/std/src/sys_common/condvar.rs

@@ -1,72 +1,62 @@
 use crate::sys::condvar as imp;
 use crate::sys_common::mutex::MovableMutex;
 use crate::time::Duration;
+use check::CondvarCheck;
+
+mod check;
 
 /// An OS-based condition variable.
-///
-/// This structure is the lowest layer possible on top of the OS-provided
-/// condition variables. It is consequently entirely unsafe to use. It is
-/// recommended to use the safer types at the top level of this crate instead of
-/// this type.
-pub struct Condvar(imp::Condvar);
+pub struct Condvar {
+    inner: Box<imp::Condvar>,
+    check: CondvarCheck,
+}
 
 impl Condvar {
     /// Creates a new condition variable for use.
-    ///
-    /// Behavior is undefined if the condition variable is moved after it is
-    /// first used with any of the functions below.
-    pub const fn new() -> Condvar {
-        Condvar(imp::Condvar::new())
-    }
-
-    /// Prepares the condition variable for use.
-    ///
-    /// This should be called once the condition variable is at a stable memory
-    /// address.
-    #[inline]
-    pub unsafe fn init(&mut self) {
-        self.0.init()
+    pub fn new() -> Self {
+        let mut c = box imp::Condvar::new();
+        unsafe { c.init() };
+        Self { inner: c, check: CondvarCheck::new() }
     }
 
     /// Signals one waiter on this condition variable to wake up.
     #[inline]
-    pub unsafe fn notify_one(&self) {
-        self.0.notify_one()
+    pub fn notify_one(&self) {
+        unsafe { self.inner.notify_one() };
     }
 
     /// Awakens all current waiters on this condition variable.
     #[inline]
-    pub unsafe fn notify_all(&self) {
-        self.0.notify_all()
+    pub fn notify_all(&self) {
+        unsafe { self.inner.notify_all() };
     }
 
     /// Waits for a signal on the specified mutex.
     ///
     /// Behavior is undefined if the mutex is not locked by the current thread.
-    /// Behavior is also undefined if more than one mutex is used concurrently
-    /// on this condition variable.
+    ///
+    /// May panic if used with more than one mutex.
     #[inline]
     pub unsafe fn wait(&self, mutex: &MovableMutex) {
-        self.0.wait(mutex.raw())
+        self.check.verify(mutex);
+        self.inner.wait(mutex.raw())
     }
 
     /// Waits for a signal on the specified mutex with a timeout duration
     /// specified by `dur` (a relative time into the future).
     ///
     /// Behavior is undefined if the mutex is not locked by the current thread.
-    /// Behavior is also undefined if more than one mutex is used concurrently
-    /// on this condition variable.
+    ///
+    /// May panic if used with more than one mutex.
     #[inline]
     pub unsafe fn wait_timeout(&self, mutex: &MovableMutex, dur: Duration) -> bool {
-        self.0.wait_timeout(mutex.raw(), dur)
+        self.check.verify(mutex);
+        self.inner.wait_timeout(mutex.raw(), dur)
     }
+}
 
-    /// Deallocates all resources associated with this condition variable.
-    ///
-    /// Behavior is undefined if there are current or will be future users of
-    /// this condition variable.
-    #[inline]
-    pub unsafe fn destroy(&self) {
-        self.0.destroy()
+impl Drop for Condvar {
+    fn drop(&mut self) {
+        unsafe { self.inner.destroy() };
     }
 }

library/std/src/sys_common/condvar/check.rs

@@ -0,0 +1,23 @@
+use crate::sync::atomic::{AtomicUsize, Ordering};
+use crate::sys::mutex as mutex_imp;
+use crate::sys_common::mutex::MovableMutex;
+
+/// A `Condvar` will check it's only ever used with the same mutex, based on
+/// its (stable) address.
+pub struct CondvarCheck {
+    addr: AtomicUsize,
+}
+
+impl CondvarCheck {
+    pub const fn new() -> Self {
+        Self { addr: AtomicUsize::new(0) }
+    }
+    pub fn verify(&self, mutex: &MovableMutex) {
+        let addr = mutex.raw() as *const mutex_imp::Mutex as usize;
+        match self.addr.compare_and_swap(0, addr, Ordering::SeqCst) {
+            0 => {}              // Stored the address
+            n if n == addr => {} // Lost a race to store the same address
+            _ => panic!("attempted to use a condition variable with two mutexes"),
+        }
+    }
+}

library/std/src/sys_common/mutex.rs

@@ -72,7 +72,7 @@ impl MovableMutex {
         Self(mutex)
     }
 
-    pub(crate) fn raw(&self) -> &imp::Mutex {
+    pub(super) fn raw(&self) -> &imp::Mutex {
         &self.0
     }