1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
use crate::cell::UnsafeCell;
use crate::mem::{forget, MaybeUninit};
use crate::sys::cvt_nz;
use crate::sys_common::lazy_box::{LazyBox, LazyInit};
struct AllocatedMutex(UnsafeCell<libc::pthread_mutex_t>);
pub struct Mutex {
inner: LazyBox<AllocatedMutex>,
}
#[inline]
pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
m.inner.0.get()
}
unsafe impl Send for AllocatedMutex {}
unsafe impl Sync for AllocatedMutex {}
impl LazyInit for AllocatedMutex {
fn init() -> Box<Self> {
let mutex = Box::new(AllocatedMutex(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER)));
// Issue #33770
//
// A pthread mutex initialized with PTHREAD_MUTEX_INITIALIZER will have
// a type of PTHREAD_MUTEX_DEFAULT, which has undefined behavior if you
// try to re-lock it from the same thread when you already hold a lock
// (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_init.html).
// This is the case even if PTHREAD_MUTEX_DEFAULT == PTHREAD_MUTEX_NORMAL
// (https://github.com/rust-lang/rust/issues/33770#issuecomment-220847521) -- in that
// case, `pthread_mutexattr_settype(PTHREAD_MUTEX_DEFAULT)` will of course be the same
// as setting it to `PTHREAD_MUTEX_NORMAL`, but not setting any mode will result in
// a Mutex where re-locking is UB.
//
// In practice, glibc takes advantage of this undefined behavior to
// implement hardware lock elision, which uses hardware transactional
// memory to avoid acquiring the lock. While a transaction is in
// progress, the lock appears to be unlocked. This isn't a problem for
// other threads since the transactional memory will abort if a conflict
// is detected, however no abort is generated when re-locking from the
// same thread.
//
// Since locking the same mutex twice will result in two aliasing &mut
// references, we instead create the mutex with type
// PTHREAD_MUTEX_NORMAL which is guaranteed to deadlock if we try to
// re-lock it from the same thread, thus avoiding undefined behavior.
unsafe {
let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
cvt_nz(libc::pthread_mutexattr_init(attr.as_mut_ptr())).unwrap();
let attr = PthreadMutexAttr(&mut attr);
cvt_nz(libc::pthread_mutexattr_settype(
attr.0.as_mut_ptr(),
libc::PTHREAD_MUTEX_NORMAL,
))
.unwrap();
cvt_nz(libc::pthread_mutex_init(mutex.0.get(), attr.0.as_ptr())).unwrap();
}
mutex
}
fn destroy(mutex: Box<Self>) {
// We're not allowed to pthread_mutex_destroy a locked mutex,
// so check first if it's unlocked.
if unsafe { libc::pthread_mutex_trylock(mutex.0.get()) == 0 } {
unsafe { libc::pthread_mutex_unlock(mutex.0.get()) };
drop(mutex);
} else {
// The mutex is locked. This happens if a MutexGuard is leaked.
// In this case, we just leak the Mutex too.
forget(mutex);
}
}
fn cancel_init(_: Box<Self>) {
// In this case, we can just drop it without any checks,
// since it cannot have been locked yet.
}
}
impl Drop for AllocatedMutex {
#[inline]
fn drop(&mut self) {
let r = unsafe { libc::pthread_mutex_destroy(self.0.get()) };
if cfg!(target_os = "dragonfly") {
// On DragonFly pthread_mutex_destroy() returns EINVAL if called on a
// mutex that was just initialized with libc::PTHREAD_MUTEX_INITIALIZER.
// Once it is used (locked/unlocked) or pthread_mutex_init() is called,
// this behaviour no longer occurs.
debug_assert!(r == 0 || r == libc::EINVAL);
} else {
debug_assert_eq!(r, 0);
}
}
}
impl Mutex {
#[inline]
pub const fn new() -> Mutex {
Mutex { inner: LazyBox::new() }
}
#[inline]
pub unsafe fn lock(&self) {
let r = libc::pthread_mutex_lock(raw(self));
debug_assert_eq!(r, 0);
}
#[inline]
pub unsafe fn unlock(&self) {
let r = libc::pthread_mutex_unlock(raw(self));
debug_assert_eq!(r, 0);
}
#[inline]
pub unsafe fn try_lock(&self) -> bool {
libc::pthread_mutex_trylock(raw(self)) == 0
}
}
pub(super) struct PthreadMutexAttr<'a>(pub &'a mut MaybeUninit<libc::pthread_mutexattr_t>);
impl Drop for PthreadMutexAttr<'_> {
fn drop(&mut self) {
unsafe {
let result = libc::pthread_mutexattr_destroy(self.0.as_mut_ptr());
debug_assert_eq!(result, 0);
}
}
}