src/sync.rs

   1 use std::collections::{hash_map::Entry, HashMap, VecDeque};
   2 use std::convert::TryFrom;
   3 use std::num::NonZeroU32;
   4 use std::ops::Not;
   5
   6 use log::trace;
   7
   8 use rustc_index::vec::{Idx, IndexVec};
   9
  10 use crate::*;
  11
  12 /// We cannot use the `newtype_index!` macro because we have to use 0 as a
  13 /// sentinel value meaning that the identifier is not assigned. This is because
  14 /// the pthreads static initializers initialize memory with zeros (see the
  15 /// `src/shims/sync.rs` file).
  16 macro_rules! declare_id {
  17     ($name: ident) => {
  18         /// 0 is used to indicate that the id was not yet assigned and,
  19         /// therefore, is not a valid identifier.
  20         #[derive(Clone, Copy, Debug, PartialOrd, Ord, PartialEq, Eq, Hash)]
  21         pub struct $name(NonZeroU32);
  22
  23         impl $name {
  24             // Panics if `id == 0`.
  25             pub fn from_u32(id: u32) -> Self {
  26                 Self(NonZeroU32::new(id).unwrap())
  27             }
  28         }
  29
  30         impl Idx for $name {
  31             fn new(idx: usize) -> Self {
  32                 // We use 0 as a sentinel value (see the comment above) and,
  33                 // therefore, need to shift by one when converting from an index
  34                 // into a vector.
  35                 let shifted_idx = u32::try_from(idx).unwrap().checked_add(1).unwrap();
  36                 $name(NonZeroU32::new(shifted_idx).unwrap())
  37             }
  38             fn index(self) -> usize {
  39                 // See the comment in `Self::new`.
  40                 // (This cannot underflow because self is NonZeroU32.)
  41                 usize::try_from(self.0.get() - 1).unwrap()
  42             }
  43         }
  44
  45         impl $name {
  46             pub fn to_u32_scalar<'tcx>(&self) -> Scalar<Tag> {
  47                 Scalar::from_u32(self.0.get())
  48             }
  49         }
  50     };
  51 }
  52
  53 declare_id!(MutexId);
  54
  55 /// The mutex state.
  56 #[derive(Default, Debug)]
  57 struct Mutex {
  58     /// The thread that currently owns the lock.
  59     owner: Option<ThreadId>,
  60     /// How many times the mutex was locked by the owner.
  61     lock_count: usize,
  62     /// The queue of threads waiting for this mutex.
  63     queue: VecDeque<ThreadId>,
  64 }
  65
  66 declare_id!(RwLockId);
  67
  68 /// The read-write lock state.
  69 #[derive(Default, Debug)]
  70 struct RwLock {
  71     /// The writer thread that currently owns the lock.
  72     writer: Option<ThreadId>,
  73     /// The readers that currently own the lock and how many times they acquired
  74     /// the lock.
  75     readers: HashMap<ThreadId, usize>,
  76     /// The queue of writer threads waiting for this lock.
  77     writer_queue: VecDeque<ThreadId>,
  78     /// The queue of reader threads waiting for this lock.
  79     reader_queue: VecDeque<ThreadId>,
  80 }
  81
  82 declare_id!(CondvarId);
  83
  84 /// A thread waiting on a conditional variable.
  85 #[derive(Debug)]
  86 struct CondvarWaiter {
  87     /// The thread that is waiting on this variable.
  88     thread: ThreadId,
  89     /// The mutex on which the thread is waiting.
  90     mutex: MutexId,
  91 }
  92
  93 /// The conditional variable state.
  94 #[derive(Default, Debug)]
  95 struct Condvar {
  96     waiters: VecDeque<CondvarWaiter>,
  97 }
  98
  99 /// The state of all synchronization variables.
 100 #[derive(Default, Debug)]
 101 pub(super) struct SynchronizationState {
 102     mutexes: IndexVec<MutexId, Mutex>,
 103     rwlocks: IndexVec<RwLockId, RwLock>,
 104     condvars: IndexVec<CondvarId, Condvar>,
 105 }
 106
 107 // Private extension trait for local helper methods
 108 impl<'mir, 'tcx: 'mir> EvalContextExtPriv<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 109 trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
 110     /// Take a reader out of the queue waiting for the lock.
 111     /// Returns `true` if some thread got the rwlock.
 112     #[inline]
 113     fn rwlock_dequeue_and_lock_reader(&mut self, id: RwLockId) -> bool {
 114         let this = self.eval_context_mut();
 115         if let Some(reader) = this.machine.threads.sync.rwlocks[id].reader_queue.pop_front() {
 116             this.unblock_thread(reader);
 117             this.rwlock_reader_lock(id, reader);
 118             true
 119         } else {
 120             false
 121         }
 122     }
 123
 124     /// Take the writer out of the queue waiting for the lock.
 125     /// Returns `true` if some thread got the rwlock.
 126     #[inline]
 127     fn rwlock_dequeue_and_lock_writer(&mut self, id: RwLockId) -> bool {
 128         let this = self.eval_context_mut();
 129         if let Some(writer) = this.machine.threads.sync.rwlocks[id].writer_queue.pop_front() {
 130             this.unblock_thread(writer);
 131             this.rwlock_writer_lock(id, writer);
 132             true
 133         } else {
 134             false
 135         }
 136     }
 137
 138     /// Take a thread out of the queue waiting for the mutex, and lock
 139     /// the mutex for it. Returns `true` if some thread has the mutex now.
 140     #[inline]
 141     fn mutex_dequeue_and_lock(&mut self, id: MutexId) -> bool {
 142         let this = self.eval_context_mut();
 143         if let Some(thread) = this.machine.threads.sync.mutexes[id].queue.pop_front() {
 144             this.unblock_thread(thread);
 145             this.mutex_lock(id, thread);
 146             true
 147         } else {
 148             false
 149         }
 150     }
 151 }
 152
 153 // Public interface to synchronization primitives. Please note that in most
 154 // cases, the function calls are infallible and it is the client's (shim
 155 // implementation's) responsibility to detect and deal with erroneous
 156 // situations.
 157 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 158 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
 159     #[inline]
 160     /// Create state for a new mutex.
 161     fn mutex_create(&mut self) -> MutexId {
 162         let this = self.eval_context_mut();
 163         this.machine.threads.sync.mutexes.push(Default::default())
 164     }
 165
 166     #[inline]
 167     /// Get the id of the thread that currently owns this lock.
 168     fn mutex_get_owner(&mut self, id: MutexId) -> ThreadId {
 169         let this = self.eval_context_ref();
 170         this.machine.threads.sync.mutexes[id].owner.unwrap()
 171     }
 172
 173     #[inline]
 174     /// Check if locked.
 175     fn mutex_is_locked(&self, id: MutexId) -> bool {
 176         let this = self.eval_context_ref();
 177         this.machine.threads.sync.mutexes[id].owner.is_some()
 178     }
 179
 180     /// Lock by setting the mutex owner and increasing the lock count.
 181     fn mutex_lock(&mut self, id: MutexId, thread: ThreadId) {
 182         let this = self.eval_context_mut();
 183         let mutex = &mut this.machine.threads.sync.mutexes[id];
 184         if let Some(current_owner) = mutex.owner {
 185             assert_eq!(thread, current_owner, "mutex already locked by another thread");
 186             assert!(
 187                 mutex.lock_count > 0,
 188                 "invariant violation: lock_count == 0 iff the thread is unlocked"
 189             );
 190         } else {
 191             mutex.owner = Some(thread);
 192         }
 193         mutex.lock_count = mutex.lock_count.checked_add(1).unwrap();
 194     }
 195
 196     /// Try unlocking by decreasing the lock count and returning the old lock
 197     /// count. If the lock count reaches 0, release the lock and potentially
 198     /// give to a new owner. If the lock was not locked by `expected_owner`,
 199     /// return `None`.
 200     fn mutex_unlock(
 201         &mut self,
 202         id: MutexId,
 203         expected_owner: ThreadId,
 204     ) -> Option<usize> {
 205         let this = self.eval_context_mut();
 206         let mutex = &mut this.machine.threads.sync.mutexes[id];
 207         if let Some(current_owner) = mutex.owner {
 208             // Mutex is locked.
 209             if current_owner != expected_owner {
 210                 // Only the owner can unlock the mutex.
 211                 return None;
 212             }
 213             let old_lock_count = mutex.lock_count;
 214             mutex.lock_count = old_lock_count
 215                 .checked_sub(1)
 216                 .expect("invariant violation: lock_count == 0 iff the thread is unlocked");
 217             if mutex.lock_count == 0 {
 218                 mutex.owner = None;
 219                 // The mutex is completely unlocked. Try transfering ownership
 220                 // to another thread.
 221                 this.mutex_dequeue_and_lock(id);
 222             }
 223             Some(old_lock_count)
 224         } else {
 225             // Mutex is not locked.
 226             None
 227         }
 228     }
 229
 230     #[inline]
 231     /// Put the thread into the queue waiting for the mutex.
 232     fn mutex_enqueue_and_block(&mut self, id: MutexId, thread: ThreadId) {
 233         let this = self.eval_context_mut();
 234         assert!(this.mutex_is_locked(id), "queing on unlocked mutex");
 235         this.machine.threads.sync.mutexes[id].queue.push_back(thread);
 236         this.block_thread(thread);
 237     }
 238
 239     #[inline]
 240     /// Create state for a new read write lock.
 241     fn rwlock_create(&mut self) -> RwLockId {
 242         let this = self.eval_context_mut();
 243         this.machine.threads.sync.rwlocks.push(Default::default())
 244     }
 245
 246     #[inline]
 247     /// Check if locked.
 248     fn rwlock_is_locked(&self, id: RwLockId) -> bool {
 249         let this = self.eval_context_ref();
 250         let rwlock = &this.machine.threads.sync.rwlocks[id];
 251         trace!(
 252             "rwlock_is_locked: {:?} writer is {:?} and there are {} reader threads (some of which could hold multiple read locks)",
 253             id, rwlock.writer, rwlock.readers.len(),
 254         );
 255         rwlock.writer.is_some()|| rwlock.readers.is_empty().not()
 256     }
 257
 258     #[inline]
 259     /// Check if write locked.
 260     fn rwlock_is_write_locked(&self, id: RwLockId) -> bool {
 261         let this = self.eval_context_ref();
 262         let rwlock = &this.machine.threads.sync.rwlocks[id];
 263         trace!("rwlock_is_write_locked: {:?} writer is {:?}", id, rwlock.writer);
 264         rwlock.writer.is_some()
 265     }
 266
 267     /// Read-lock the lock by adding the `reader` the list of threads that own
 268     /// this lock.
 269     fn rwlock_reader_lock(&mut self, id: RwLockId, reader: ThreadId) {
 270         let this = self.eval_context_mut();
 271         assert!(!this.rwlock_is_write_locked(id), "the lock is write locked");
 272         trace!("rwlock_reader_lock: {:?} now also held (one more time) by {:?}", id, reader);
 273         let count = this.machine.threads.sync.rwlocks[id].readers.entry(reader).or_insert(0);
 274         *count = count.checked_add(1).expect("the reader counter overflowed");
 275     }
 276
 277     /// Try read-unlock the lock for `reader` and potentially give the lock to a new owner.
 278     /// Returns `true` if succeeded, `false` if this `reader` did not hold the lock.
 279     fn rwlock_reader_unlock(&mut self, id: RwLockId, reader: ThreadId) -> bool {
 280         let this = self.eval_context_mut();
 281         match this.machine.threads.sync.rwlocks[id].readers.entry(reader) {
 282             Entry::Occupied(mut entry) => {
 283                 let count = entry.get_mut();
 284                 assert!(*count > 0, "rwlock locked with count == 0");
 285                 *count -= 1;
 286                 if *count == 0 {
 287                     trace!("rwlock_reader_unlock: {:?} no longer held by {:?}", id, reader);
 288                     entry.remove();
 289                 } else {
 290                     trace!("rwlock_reader_unlock: {:?} held one less time by {:?}", id, reader);
 291                 }
 292             }
 293             Entry::Vacant(_) => return false, // we did not even own this lock
 294         }
 295         // The thread was a reader. If the lock is not held any more, give it to a writer.
 296         if this.rwlock_is_locked(id).not() {
 297             this.rwlock_dequeue_and_lock_writer(id);
 298         }
 299         true
 300     }
 301
 302     #[inline]
 303     /// Put the reader in the queue waiting for the lock and block it.
 304     fn rwlock_enqueue_and_block_reader(
 305         &mut self,
 306         id: RwLockId,
 307         reader: ThreadId,
 308     ) {
 309         let this = self.eval_context_mut();
 310         assert!(this.rwlock_is_write_locked(id), "read-queueing on not write locked rwlock");
 311         this.machine.threads.sync.rwlocks[id].reader_queue.push_back(reader);
 312         this.block_thread(reader);
 313     }
 314
 315     #[inline]
 316     /// Lock by setting the writer that owns the lock.
 317     fn rwlock_writer_lock(&mut self, id: RwLockId, writer: ThreadId) {
 318         let this = self.eval_context_mut();
 319         assert!(!this.rwlock_is_locked(id), "the rwlock is already locked");
 320         trace!("rwlock_writer_lock: {:?} now held by {:?}", id, writer);
 321         this.machine.threads.sync.rwlocks[id].writer = Some(writer);
 322     }
 323
 324     #[inline]
 325     /// Try to unlock by removing the writer.
 326     fn rwlock_writer_unlock(&mut self, id: RwLockId, expected_writer: ThreadId) -> bool {
 327         let this = self.eval_context_mut();
 328         let rwlock = &mut this.machine.threads.sync.rwlocks[id];
 329         if let Some(current_writer) = rwlock.writer {
 330             if current_writer != expected_writer {
 331                 // Only the owner can unlock the rwlock.
 332                 return false;
 333             }
 334             rwlock.writer = None;
 335             trace!("rwlock_writer_unlock: {:?} unlocked by {:?}", id, expected_writer);
 336             // The thread was a writer.
 337             //
 338             // We are prioritizing writers here against the readers. As a
 339             // result, not only readers can starve writers, but also writers can
 340             // starve readers.
 341             if this.rwlock_dequeue_and_lock_writer(id) {
 342                 // Someone got the write lock, nice.
 343             } else {
 344                 // Give the lock to all readers.
 345                 while this.rwlock_dequeue_and_lock_reader(id) {
 346                     // Rinse and repeat.
 347                 }
 348             }
 349             true
 350         } else {
 351             false
 352         }
 353     }
 354
 355     #[inline]
 356     /// Put the writer in the queue waiting for the lock.
 357     fn rwlock_enqueue_and_block_writer(
 358         &mut self,
 359         id: RwLockId,
 360         writer: ThreadId,
 361     ) {
 362         let this = self.eval_context_mut();
 363         assert!(this.rwlock_is_locked(id), "write-queueing on unlocked rwlock");
 364         this.machine.threads.sync.rwlocks[id].writer_queue.push_back(writer);
 365         this.block_thread(writer);
 366     }
 367
 368     #[inline]
 369     /// Create state for a new conditional variable.
 370     fn condvar_create(&mut self) -> CondvarId {
 371         let this = self.eval_context_mut();
 372         this.machine.threads.sync.condvars.push(Default::default())
 373     }
 374
 375     #[inline]
 376     /// Is the conditional variable awaited?
 377     fn condvar_is_awaited(&mut self, id: CondvarId) -> bool {
 378         let this = self.eval_context_mut();
 379         !this.machine.threads.sync.condvars[id].waiters.is_empty()
 380     }
 381
 382     /// Mark that the thread is waiting on the conditional variable.
 383     fn condvar_wait(&mut self, id: CondvarId, thread: ThreadId, mutex: MutexId) {
 384         let this = self.eval_context_mut();
 385         let waiters = &mut this.machine.threads.sync.condvars[id].waiters;
 386         assert!(waiters.iter().all(|waiter| waiter.thread != thread), "thread is already waiting");
 387         waiters.push_back(CondvarWaiter { thread, mutex });
 388     }
 389
 390     /// Wake up some thread (if there is any) sleeping on the conditional
 391     /// variable.
 392     fn condvar_signal(&mut self, id: CondvarId) -> Option<(ThreadId, MutexId)> {
 393         let this = self.eval_context_mut();
 394         this.machine.threads.sync.condvars[id]
 395             .waiters
 396             .pop_front()
 397             .map(|waiter| (waiter.thread, waiter.mutex))
 398     }
 399
 400     #[inline]
 401     /// Remove the thread from the queue of threads waiting on this conditional variable.
 402     fn condvar_remove_waiter(&mut self, id: CondvarId, thread: ThreadId) {
 403         let this = self.eval_context_mut();
 404         this.machine.threads.sync.condvars[id].waiters.retain(|waiter| waiter.thread != thread);
 405     }
 406 }