library/std/src/sys/unix/locks/fuchsia_mutex.rs

   1 //! A priority inheriting mutex for Fuchsia.
   2 //!
   3 //! This is a port of the [mutex in Fuchsia's libsync]. Contrary to the original,
   4 //! it does not abort the process when reentrant locking is detected, but deadlocks.
   5 //!
   6 //! Priority inheritance is achieved by storing the owning thread's handle in an
   7 //! atomic variable. Fuchsia's futex operations support setting an owner thread
   8 //! for a futex, which can boost that thread's priority while the futex is waited
   9 //! upon.
  10 //!
  11 //! libsync is licenced under the following BSD-style licence:
  12 //!
  13 //! Copyright 2016 The Fuchsia Authors.
  14 //!
  15 //! Redistribution and use in source and binary forms, with or without
  16 //! modification, are permitted provided that the following conditions are
  17 //! met:
  18 //!
  19 //!    * Redistributions of source code must retain the above copyright
  20 //!      notice, this list of conditions and the following disclaimer.
  21 //!    * Redistributions in binary form must reproduce the above
  22 //!      copyright notice, this list of conditions and the following
  23 //!      disclaimer in the documentation and/or other materials provided
  24 //!      with the distribution.
  25 //!
  26 //! THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  27 //! "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  28 //! LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  29 //! A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  30 //! OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  31 //! SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  32 //! LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  33 //! DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  34 //! THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  35 //! (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  36 //! OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  37 //!
  38 //! [mutex in Fuchsia's libsync]: https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/system/ulib/sync/mutex.c
  39
  40 use crate::sync::atomic::{
  41     AtomicU32,
  42     Ordering::{Acquire, Relaxed, Release},
  43 };
  44 use crate::sys::futex::zircon::{
  45     zx_futex_wait, zx_futex_wake_single_owner, zx_handle_t, zx_thread_self, ZX_ERR_BAD_HANDLE,
  46     ZX_ERR_BAD_STATE, ZX_ERR_INVALID_ARGS, ZX_ERR_TIMED_OUT, ZX_ERR_WRONG_TYPE, ZX_OK,
  47     ZX_TIME_INFINITE,
  48 };
  49
  50 // The lowest two bits of a `zx_handle_t` are always set, so the lowest bit is used to mark the
  51 // mutex as contested by clearing it.
  52 const CONTESTED_BIT: u32 = 1;
  53 // This can never be a valid `zx_handle_t`.
  54 const UNLOCKED: u32 = 0;
  55
  56 pub struct Mutex {
  57     futex: AtomicU32,
  58 }
  59
  60 #[inline]
  61 fn to_state(owner: zx_handle_t) -> u32 {
  62     owner
  63 }
  64
  65 #[inline]
  66 fn to_owner(state: u32) -> zx_handle_t {
  67     state | CONTESTED_BIT
  68 }
  69
  70 #[inline]
  71 fn is_contested(state: u32) -> bool {
  72     state & CONTESTED_BIT == 0
  73 }
  74
  75 #[inline]
  76 fn mark_contested(state: u32) -> u32 {
  77     state & !CONTESTED_BIT
  78 }
  79
  80 impl Mutex {
  81     #[inline]
  82     pub const fn new() -> Mutex {
  83         Mutex { futex: AtomicU32::new(UNLOCKED) }
  84     }
  85
  86     #[inline]
  87     pub fn try_lock(&self) -> bool {
  88         let thread_self = unsafe { zx_thread_self() };
  89         self.futex.compare_exchange(UNLOCKED, to_state(thread_self), Acquire, Relaxed).is_ok()
  90     }
  91
  92     #[inline]
  93     pub fn lock(&self) {
  94         let thread_self = unsafe { zx_thread_self() };
  95         if let Err(state) =
  96             self.futex.compare_exchange(UNLOCKED, to_state(thread_self), Acquire, Relaxed)
  97         {
  98             unsafe {
  99                 self.lock_contested(state, thread_self);
 100             }
 101         }
 102     }
 103
 104     /// # Safety
 105     /// `thread_self` must be the handle for the current thread.
 106     #[cold]
 107     unsafe fn lock_contested(&self, mut state: u32, thread_self: zx_handle_t) {
 108         let owned_state = mark_contested(to_state(thread_self));
 109         loop {
 110             // Mark the mutex as contested if it is not already.
 111             let contested = mark_contested(state);
 112             if is_contested(state)
 113                 || self.futex.compare_exchange(state, contested, Relaxed, Relaxed).is_ok()
 114             {
 115                 // The mutex has been marked as contested, wait for the state to change.
 116                 unsafe {
 117                     match zx_futex_wait(
 118                         &self.futex,
 119                         AtomicU32::new(contested),
 120                         to_owner(state),
 121                         ZX_TIME_INFINITE,
 122                     ) {
 123                         ZX_OK | ZX_ERR_BAD_STATE | ZX_ERR_TIMED_OUT => (),
 124                         // Note that if a thread handle is reused after its associated thread
 125                         // exits without unlocking the mutex, an arbitrary thread's priority
 126                         // could be boosted by the wait, but there is currently no way to
 127                         // prevent that.
 128                         ZX_ERR_INVALID_ARGS | ZX_ERR_BAD_HANDLE | ZX_ERR_WRONG_TYPE => {
 129                             panic!(
 130                                 "either the current thread is trying to lock a mutex it has
 131                                 already locked, or the previous owner did not unlock the mutex
 132                                 before exiting"
 133                             )
 134                         }
 135                         error => panic!("unexpected error in zx_futex_wait: {error}"),
 136                     }
 137                 }
 138             }
 139
 140             // The state has changed or a wakeup occurred, try to lock the mutex.
 141             match self.futex.compare_exchange(UNLOCKED, owned_state, Acquire, Relaxed) {
 142                 Ok(_) => return,
 143                 Err(updated) => state = updated,
 144             }
 145         }
 146     }
 147
 148     #[inline]
 149     pub unsafe fn unlock(&self) {
 150         if is_contested(self.futex.swap(UNLOCKED, Release)) {
 151             // The woken thread will mark the mutex as contested again,
 152             // and return here, waking until there are no waiters left,
 153             // in which case this is a noop.
 154             self.wake();
 155         }
 156     }
 157
 158     #[cold]
 159     fn wake(&self) {
 160         unsafe {
 161             zx_futex_wake_single_owner(&self.futex);
 162         }
 163     }
 164 }