library/std/src/sys/cloudabi/rwlock.rs

   1 use crate::mem;
   2 use crate::mem::MaybeUninit;
   3 use crate::sync::atomic::{AtomicU32, Ordering};
   4 use crate::sys::cloudabi::abi;
   5
   6 extern "C" {
   7     #[thread_local]
   8     static __pthread_thread_id: abi::tid;
   9 }
  10
  11 #[thread_local]
  12 static mut RDLOCKS_ACQUIRED: u32 = 0;
  13
  14 pub struct RWLock {
  15     lock: AtomicU32,
  16 }
  17
  18 pub unsafe fn raw(r: &RWLock) -> &AtomicU32 {
  19     &r.lock
  20 }
  21
  22 unsafe impl Send for RWLock {}
  23 unsafe impl Sync for RWLock {}
  24
  25 impl RWLock {
  26     pub const fn new() -> RWLock {
  27         RWLock { lock: AtomicU32::new(abi::LOCK_UNLOCKED.0) }
  28     }
  29
  30     pub unsafe fn try_read(&self) -> bool {
  31         let mut old = abi::LOCK_UNLOCKED.0;
  32         while let Err(cur) =
  33             self.lock.compare_exchange_weak(old, old + 1, Ordering::Acquire, Ordering::Relaxed)
  34         {
  35             if (cur & abi::LOCK_WRLOCKED.0) != 0 {
  36                 // Another thread already has a write lock.
  37                 assert_ne!(
  38                     old & !abi::LOCK_KERNEL_MANAGED.0,
  39                     __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0,
  40                     "Attempted to acquire a read lock while holding a write lock"
  41                 );
  42                 return false;
  43             } else if (old & abi::LOCK_KERNEL_MANAGED.0) != 0 && RDLOCKS_ACQUIRED == 0 {
  44                 // Lock has threads waiting for the lock. Only acquire
  45                 // the lock if we have already acquired read locks. In
  46                 // that case, it is justified to acquire this lock to
  47                 // prevent a deadlock.
  48                 return false;
  49             }
  50             old = cur;
  51         }
  52
  53         RDLOCKS_ACQUIRED += 1;
  54         true
  55     }
  56
  57     pub unsafe fn read(&self) {
  58         if !self.try_read() {
  59             // Call into the kernel to acquire a read lock.
  60             let subscription = abi::subscription {
  61                 type_: abi::eventtype::LOCK_RDLOCK,
  62                 union: abi::subscription_union {
  63                     lock: abi::subscription_lock {
  64                         lock: &self.lock as *const AtomicU32 as *mut abi::lock,
  65                         lock_scope: abi::scope::PRIVATE,
  66                     },
  67                 },
  68                 ..mem::zeroed()
  69             };
  70             let mut event = MaybeUninit::<abi::event>::uninit();
  71             let mut nevents = MaybeUninit::<usize>::uninit();
  72             let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
  73             assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire read lock");
  74             let event = event.assume_init();
  75             assert_eq!(event.error, abi::errno::SUCCESS, "Failed to acquire read lock");
  76
  77             RDLOCKS_ACQUIRED += 1;
  78         }
  79     }
  80
  81     pub unsafe fn read_unlock(&self) {
  82         // Perform a read unlock. We can do this in userspace, except when
  83         // other threads are blocked and we are performing the last unlock.
  84         // In that case, call into the kernel.
  85         //
  86         // Other threads may attempt to increment the read lock count,
  87         // meaning that the call into the kernel could be spurious. To
  88         // prevent this from happening, upgrade to a write lock first. This
  89         // allows us to call into the kernel, having the guarantee that the
  90         // lock value will not change in the meantime.
  91         assert!(RDLOCKS_ACQUIRED > 0, "Bad lock count");
  92         let mut old = 1;
  93         loop {
  94             if old == 1 | abi::LOCK_KERNEL_MANAGED.0 {
  95                 // Last read lock while threads are waiting. Attempt to upgrade
  96                 // to a write lock before calling into the kernel to unlock.
  97                 if let Err(cur) = self.lock.compare_exchange_weak(
  98                     old,
  99                     __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0 | abi::LOCK_KERNEL_MANAGED.0,
 100                     Ordering::Acquire,
 101                     Ordering::Relaxed,
 102                 ) {
 103                     old = cur;
 104                 } else {
 105                     // Call into the kernel to unlock.
 106                     let ret = abi::lock_unlock(
 107                         &self.lock as *const AtomicU32 as *mut abi::lock,
 108                         abi::scope::PRIVATE,
 109                     );
 110                     assert_eq!(ret, abi::errno::SUCCESS, "Failed to write unlock a rwlock");
 111                     break;
 112                 }
 113             } else {
 114                 // No threads waiting or not the last read lock. Just decrement
 115                 // the read lock count.
 116                 assert_ne!(old & !abi::LOCK_KERNEL_MANAGED.0, 0, "This rwlock is not locked");
 117                 assert_eq!(
 118                     old & abi::LOCK_WRLOCKED.0,
 119                     0,
 120                     "Attempted to read-unlock a write-locked rwlock"
 121                 );
 122                 if let Err(cur) = self.lock.compare_exchange_weak(
 123                     old,
 124                     old - 1,
 125                     Ordering::Acquire,
 126                     Ordering::Relaxed,
 127                 ) {
 128                     old = cur;
 129                 } else {
 130                     break;
 131                 }
 132             }
 133         }
 134
 135         RDLOCKS_ACQUIRED -= 1;
 136     }
 137
 138     pub unsafe fn try_write(&self) -> bool {
 139         // Attempt to acquire the lock.
 140         if let Err(old) = self.lock.compare_exchange(
 141             abi::LOCK_UNLOCKED.0,
 142             __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0,
 143             Ordering::Acquire,
 144             Ordering::Relaxed,
 145         ) {
 146             // Failure. Crash upon recursive acquisition.
 147             assert_ne!(
 148                 old & !abi::LOCK_KERNEL_MANAGED.0,
 149                 __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0,
 150                 "Attempted to recursive write-lock a rwlock",
 151             );
 152             false
 153         } else {
 154             // Success.
 155             true
 156         }
 157     }
 158
 159     pub unsafe fn write(&self) {
 160         if !self.try_write() {
 161             // Call into the kernel to acquire a write lock.
 162             let subscription = abi::subscription {
 163                 type_: abi::eventtype::LOCK_WRLOCK,
 164                 union: abi::subscription_union {
 165                     lock: abi::subscription_lock {
 166                         lock: &self.lock as *const AtomicU32 as *mut abi::lock,
 167                         lock_scope: abi::scope::PRIVATE,
 168                     },
 169                 },
 170                 ..mem::zeroed()
 171             };
 172             let mut event = MaybeUninit::<abi::event>::uninit();
 173             let mut nevents = MaybeUninit::<usize>::uninit();
 174             let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
 175             assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire write lock");
 176             let event = event.assume_init();
 177             assert_eq!(event.error, abi::errno::SUCCESS, "Failed to acquire write lock");
 178         }
 179     }
 180
 181     pub unsafe fn write_unlock(&self) {
 182         assert_eq!(
 183             self.lock.load(Ordering::Relaxed) & !abi::LOCK_KERNEL_MANAGED.0,
 184             __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0,
 185             "This rwlock is not write-locked by this thread"
 186         );
 187
 188         if !self
 189             .lock
 190             .compare_exchange(
 191                 __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0,
 192                 abi::LOCK_UNLOCKED.0,
 193                 Ordering::Release,
 194                 Ordering::Relaxed,
 195             )
 196             .is_ok()
 197         {
 198             // Lock is managed by kernelspace. Call into the kernel
 199             // to unblock waiting threads.
 200             let ret = abi::lock_unlock(
 201                 &self.lock as *const AtomicU32 as *mut abi::lock,
 202                 abi::scope::PRIVATE,
 203             );
 204             assert_eq!(ret, abi::errno::SUCCESS, "Failed to write unlock a rwlock");
 205         }
 206     }
 207
 208     pub unsafe fn destroy(&self) {
 209         assert_eq!(
 210             self.lock.load(Ordering::Relaxed),
 211             abi::LOCK_UNLOCKED.0,
 212             "Attempted to destroy locked rwlock"
 213         );
 214     }
 215 }