diff options
Diffstat (limited to 'library/std/src/sys/cloudabi/rwlock.rs')
| -rw-r--r-- | library/std/src/sys/cloudabi/rwlock.rs | 218 |
1 files changed, 218 insertions, 0 deletions
diff --git a/library/std/src/sys/cloudabi/rwlock.rs b/library/std/src/sys/cloudabi/rwlock.rs new file mode 100644 index 00000000000..b8af5af1d70 --- /dev/null +++ b/library/std/src/sys/cloudabi/rwlock.rs @@ -0,0 +1,218 @@ +use crate::cell::UnsafeCell; +use crate::mem; +use crate::mem::MaybeUninit; +use crate::sync::atomic::{AtomicU32, Ordering}; +use crate::sys::cloudabi::abi; + +extern "C" { + #[thread_local] + static __pthread_thread_id: abi::tid; +} + +#[thread_local] +static mut RDLOCKS_ACQUIRED: u32 = 0; + +pub struct RWLock { + lock: UnsafeCell<AtomicU32>, +} + +pub unsafe fn raw(r: &RWLock) -> *mut AtomicU32 { + r.lock.get() +} + +unsafe impl Send for RWLock {} +unsafe impl Sync for RWLock {} + +const NEW: RWLock = RWLock { lock: UnsafeCell::new(AtomicU32::new(abi::LOCK_UNLOCKED.0)) }; + +impl RWLock { + pub const fn new() -> RWLock { + NEW + } + + pub unsafe fn try_read(&self) -> bool { + let lock = self.lock.get(); + let mut old = abi::LOCK_UNLOCKED.0; + while let Err(cur) = + (*lock).compare_exchange_weak(old, old + 1, Ordering::Acquire, Ordering::Relaxed) + { + if (cur & abi::LOCK_WRLOCKED.0) != 0 { + // Another thread already has a write lock. + assert_ne!( + old & !abi::LOCK_KERNEL_MANAGED.0, + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0, + "Attempted to acquire a read lock while holding a write lock" + ); + return false; + } else if (old & abi::LOCK_KERNEL_MANAGED.0) != 0 && RDLOCKS_ACQUIRED == 0 { + // Lock has threads waiting for the lock. Only acquire + // the lock if we have already acquired read locks. In + // that case, it is justified to acquire this lock to + // prevent a deadlock. + return false; + } + old = cur; + } + + RDLOCKS_ACQUIRED += 1; + true + } + + pub unsafe fn read(&self) { + if !self.try_read() { + // Call into the kernel to acquire a read lock. + let lock = self.lock.get(); + let subscription = abi::subscription { + type_: abi::eventtype::LOCK_RDLOCK, + union: abi::subscription_union { + lock: abi::subscription_lock { + lock: lock as *mut abi::lock, + lock_scope: abi::scope::PRIVATE, + }, + }, + ..mem::zeroed() + }; + let mut event = MaybeUninit::<abi::event>::uninit(); + let mut nevents = MaybeUninit::<usize>::uninit(); + let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr()); + assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire read lock"); + let event = event.assume_init(); + assert_eq!(event.error, abi::errno::SUCCESS, "Failed to acquire read lock"); + + RDLOCKS_ACQUIRED += 1; + } + } + + pub unsafe fn read_unlock(&self) { + // Perform a read unlock. We can do this in userspace, except when + // other threads are blocked and we are performing the last unlock. + // In that case, call into the kernel. + // + // Other threads may attempt to increment the read lock count, + // meaning that the call into the kernel could be spurious. To + // prevent this from happening, upgrade to a write lock first. This + // allows us to call into the kernel, having the guarantee that the + // lock value will not change in the meantime. + assert!(RDLOCKS_ACQUIRED > 0, "Bad lock count"); + let mut old = 1; + loop { + let lock = self.lock.get(); + if old == 1 | abi::LOCK_KERNEL_MANAGED.0 { + // Last read lock while threads are waiting. Attempt to upgrade + // to a write lock before calling into the kernel to unlock. + if let Err(cur) = (*lock).compare_exchange_weak( + old, + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0 | abi::LOCK_KERNEL_MANAGED.0, + Ordering::Acquire, + Ordering::Relaxed, + ) { + old = cur; + } else { + // Call into the kernel to unlock. + let ret = abi::lock_unlock(lock as *mut abi::lock, abi::scope::PRIVATE); + assert_eq!(ret, abi::errno::SUCCESS, "Failed to write unlock a rwlock"); + break; + } + } else { + // No threads waiting or not the last read lock. Just decrement + // the read lock count. + assert_ne!(old & !abi::LOCK_KERNEL_MANAGED.0, 0, "This rwlock is not locked"); + assert_eq!( + old & abi::LOCK_WRLOCKED.0, + 0, + "Attempted to read-unlock a write-locked rwlock" + ); + if let Err(cur) = (*lock).compare_exchange_weak( + old, + old - 1, + Ordering::Acquire, + Ordering::Relaxed, + ) { + old = cur; + } else { + break; + } + } + } + + RDLOCKS_ACQUIRED -= 1; + } + + pub unsafe fn try_write(&self) -> bool { + // Attempt to acquire the lock. + let lock = self.lock.get(); + if let Err(old) = (*lock).compare_exchange( + abi::LOCK_UNLOCKED.0, + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0, + Ordering::Acquire, + Ordering::Relaxed, + ) { + // Failure. Crash upon recursive acquisition. + assert_ne!( + old & !abi::LOCK_KERNEL_MANAGED.0, + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0, + "Attempted to recursive write-lock a rwlock", + ); + false + } else { + // Success. + true + } + } + + pub unsafe fn write(&self) { + if !self.try_write() { + // Call into the kernel to acquire a write lock. + let lock = self.lock.get(); + let subscription = abi::subscription { + type_: abi::eventtype::LOCK_WRLOCK, + union: abi::subscription_union { + lock: abi::subscription_lock { + lock: lock as *mut abi::lock, + lock_scope: abi::scope::PRIVATE, + }, + }, + ..mem::zeroed() + }; + let mut event = MaybeUninit::<abi::event>::uninit(); + let mut nevents = MaybeUninit::<usize>::uninit(); + let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr()); + assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire write lock"); + let event = event.assume_init(); + assert_eq!(event.error, abi::errno::SUCCESS, "Failed to acquire write lock"); + } + } + + pub unsafe fn write_unlock(&self) { + let lock = self.lock.get(); + assert_eq!( + (*lock).load(Ordering::Relaxed) & !abi::LOCK_KERNEL_MANAGED.0, + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0, + "This rwlock is not write-locked by this thread" + ); + + if !(*lock) + .compare_exchange( + __pthread_thread_id.0 | abi::LOCK_WRLOCKED.0, + abi::LOCK_UNLOCKED.0, + Ordering::Release, + Ordering::Relaxed, + ) + .is_ok() + { + // Lock is managed by kernelspace. Call into the kernel + // to unblock waiting threads. + let ret = abi::lock_unlock(lock as *mut abi::lock, abi::scope::PRIVATE); + assert_eq!(ret, abi::errno::SUCCESS, "Failed to write unlock a rwlock"); + } + } + + pub unsafe fn destroy(&self) { + let lock = self.lock.get(); + assert_eq!( + (*lock).load(Ordering::Relaxed), + abi::LOCK_UNLOCKED.0, + "Attempted to destroy locked rwlock" + ); + } +} |