1 files changed, 94 insertions, 79 deletions

diff --git a/ctr-std/src/sys/unix/rwlock.rs b/ctr-std/src/sys/unix/rwlock.rs
index 4a802e9..c754d5b 100644
--- a/ctr-std/src/sys/unix/rwlock.rs
+++ b/ctr-std/src/sys/unix/rwlock.rs
@@ -1,4 +1,4 @@
-// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,16 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use libc;
 use cell::UnsafeCell;
-use super::mutex::Mutex;
-use super::condvar::Condvar;
+use sync::atomic::{AtomicUsize, Ordering};
 
-// A simple read-preferring RWLock implementation that I found on wikipedia <.<
 pub struct RWLock {
-    mutex: Mutex,
-    cvar: Condvar,
-    reader_count: UnsafeCell<u32>, 
-    writer_active: UnsafeCell<bool>,
+    inner: UnsafeCell<libc::pthread_rwlock_t>,
+    write_locked: UnsafeCell<bool>,
+    num_readers: AtomicUsize,
 }
 
 unsafe impl Send for RWLock {}
@@ -26,102 +24,119 @@ unsafe impl Sync for RWLock {}
 impl RWLock {
     pub const fn new() -> RWLock {
         RWLock {
-            mutex: Mutex::new(),
-            cvar: Condvar::new(),
-            reader_count: UnsafeCell::new(0),
-            writer_active: UnsafeCell::new(false),
+            inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER),
+            write_locked: UnsafeCell::new(false),
+            num_readers: AtomicUsize::new(0),
         }
     }
-
     #[inline]
     pub unsafe fn read(&self) {
-        self.mutex.lock();
-
-        while *self.writer_active.get() {
-            self.cvar.wait(&self.mutex);
+        let r = libc::pthread_rwlock_rdlock(self.inner.get());
+
+        // According to the pthread_rwlock_rdlock spec, this function **may**
+        // fail with EDEADLK if a deadlock is detected. On the other hand
+        // pthread mutexes will *never* return EDEADLK if they are initialized
+        // as the "fast" kind (which ours always are). As a result, a deadlock
+        // situation may actually return from the call to pthread_rwlock_rdlock
+        // instead of blocking forever (as mutexes and Windows rwlocks do). Note
+        // that not all unix implementations, however, will return EDEADLK for
+        // their rwlocks.
+        //
+        // We roughly maintain the deadlocking behavior by panicking to ensure
+        // that this lock acquisition does not succeed.
+        //
+        // We also check whether this lock is already write locked. This
+        // is only possible if it was write locked by the current thread and
+        // the implementation allows recursive locking. The POSIX standard
+        // doesn't require recursively locking a rwlock to deadlock, but we can't
+        // allow that because it could lead to aliasing issues.
+        if r == libc::EAGAIN {
+            panic!("rwlock maximum reader count exceeded");
+        } else if r == libc::EDEADLK || *self.write_locked.get() {
+            if r == 0 {
+                self.raw_unlock();
+            }
+            panic!("rwlock read lock would result in deadlock");
+        } else {
+            debug_assert_eq!(r, 0);
+            self.num_readers.fetch_add(1, Ordering::Relaxed);
         }
-
-        assert!(*self.reader_count.get() != u32::max_value());
-        *self.reader_count.get() += 1;
-
-        self.mutex.unlock();
     }
-
     #[inline]
     pub unsafe fn try_read(&self) -> bool {
-        if !self.mutex.try_lock() {
-            return false
-        }
-
-        while *self.writer_active.get() {
-            self.cvar.wait(&self.mutex);
+        let r = libc::pthread_rwlock_tryrdlock(self.inner.get());
+        if r == 0 {
+            if *self.write_locked.get() {
+                self.raw_unlock();
+                false
+            } else {
+                self.num_readers.fetch_add(1, Ordering::Relaxed);
+                true
+            }
+        } else {
+            false
         }
-
-        assert!(*self.reader_count.get() != u32::max_value());
-        *self.reader_count.get() += 1;
-
-        self.mutex.unlock();
-        true
     }
-
     #[inline]
     pub unsafe fn write(&self) {
-        self.mutex.lock();
-
-        while *self.writer_active.get() || *self.reader_count.get() > 0 {
-            self.cvar.wait(&self.mutex);
+        let r = libc::pthread_rwlock_wrlock(self.inner.get());
+        // See comments above for why we check for EDEADLK and write_locked. We
+        // also need to check that num_readers is 0.
+        if r == libc::EDEADLK || *self.write_locked.get() ||
+           self.num_readers.load(Ordering::Relaxed) != 0 {
+            if r == 0 {
+                self.raw_unlock();
+            }
+            panic!("rwlock write lock would result in deadlock");
+        } else {
+            debug_assert_eq!(r, 0);
         }
-
-        *self.writer_active.get() = true;
-
-        self.mutex.unlock();
+        *self.write_locked.get() = true;
     }
-
     #[inline]
     pub unsafe fn try_write(&self) -> bool {
-        if !self.mutex.try_lock() {
-            return false;
-        }
-
-        while *self.writer_active.get() || *self.reader_count.get() > 0 {
-            self.cvar.wait(&self.mutex);
+        let r = libc::pthread_rwlock_trywrlock(self.inner.get());
+        if r == 0 {
+            if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 {
+                self.raw_unlock();
+                false
+            } else {
+                *self.write_locked.get() = true;
+                true
+            }
+        } else {
+            false
         }
-
-        *self.writer_active.get() = true;
-
-        self.mutex.unlock();
-        true
     }
-
+    #[inline]
+    unsafe fn raw_unlock(&self) {
+        let r = libc::pthread_rwlock_unlock(self.inner.get());
+        debug_assert_eq!(r, 0);
+    }
     #[inline]
     pub unsafe fn read_unlock(&self) {
-        self.mutex.lock();
-
-        *self.reader_count.get() -= 1;
-
-        if *self.reader_count.get() == 0 {
-            self.cvar.notify_one()
-        }
-
-        self.mutex.unlock();
+        debug_assert!(!*self.write_locked.get());
+        self.num_readers.fetch_sub(1, Ordering::Relaxed);
+        self.raw_unlock();
     }
-
     #[inline]
     pub unsafe fn write_unlock(&self) {
-        self.mutex.lock();
-
-        *self.writer_active.get() = false;
-
-        self.cvar.notify_all();
-
-        self.mutex.unlock();
+        debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0);
+        debug_assert!(*self.write_locked.get());
+        *self.write_locked.get() = false;
+        self.raw_unlock();
     }
-
     #[inline]
     pub unsafe fn destroy(&self) {
-        self.mutex.destroy();
-        self.cvar.destroy();
-        *self.reader_count.get() = 0;
-        *self.writer_active.get() = false;
+        let r = libc::pthread_rwlock_destroy(self.inner.get());
+        // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
+        // rwlock that was just initialized with
+        // libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
+        // or pthread_rwlock_init() is called, this behaviour no longer occurs.
+        if cfg!(target_os = "dragonfly") {
+            debug_assert!(r == 0 || r == libc::EINVAL);
+        } else {
+            debug_assert_eq!(r, 0);
+        }
     }
 }