Revert "Set num_threads to 50 on 32-bit hppa in two libgomp loop tests"

[official-gcc.git] / libgo / go / sync / rwmutex.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (
        "internal/race"
        "sync/atomic"
        "unsafe"
)

// There is a modified copy of this file in runtime/rwmutex.go.
// If you make any changes here, see if you should make them there.

// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// A RWMutex must not be copied after first use.
//
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released. In particular, this prohibits
// recursive read locking. This is to ensure that the lock eventually becomes
// available; a blocked Lock call excludes new readers from acquiring the
// lock.
type RWMutex struct {
        w           Mutex  // held if there are pending writers
        writerSem   uint32 // semaphore for writers to wait for completing readers
        readerSem   uint32 // semaphore for readers to wait for completing writers
        readerCount int32  // number of pending readers
        readerWait  int32  // number of departing readers
}

const rwmutexMaxReaders = 1 << 30

// Happens-before relationships are indicated to the race detector via:
// - Unlock  -> Lock:  readerSem
// - Unlock  -> RLock: readerSem
// - RUnlock -> Lock:  writerSem
//
// The methods below temporarily disable handling of race synchronization
// events in order to provide the more precise model above to the race
// detector.
//
// For example, atomic.AddInt32 in RLock should not appear to provide
// acquire-release semantics, which would incorrectly synchronize racing
// readers, thus potentially missing races.

// RLock locks rw for reading.
//
// It should not be used for recursive read locking; a blocked Lock
// call excludes new readers from acquiring the lock. See the
// documentation on the RWMutex type.
func (rw *RWMutex) RLock() {
        if race.Enabled {
                _ = rw.w.state
                race.Disable()
        }
        if atomic.AddInt32(&rw.readerCount, 1) < 0 {
                // A writer is pending, wait for it.
                runtime_SemacquireMutex(&rw.readerSem, false, 0)
        }
        if race.Enabled {
                race.Enable()
                race.Acquire(unsafe.Pointer(&rw.readerSem))
        }
}

// TryRLock tries to lock rw for reading and reports whether it succeeded.
//
// Note that while correct uses of TryRLock do exist, they are rare,
// and use of TryRLock is often a sign of a deeper problem
// in a particular use of mutexes.
func (rw *RWMutex) TryRLock() bool {
        if race.Enabled {
                _ = rw.w.state
                race.Disable()
        }
        for {
                c := atomic.LoadInt32(&rw.readerCount)
                if c < 0 {
                        if race.Enabled {
                                race.Enable()
                        }
                        return false
                }
                if atomic.CompareAndSwapInt32(&rw.readerCount, c, c+1) {
                        if race.Enabled {
                                race.Enable()
                                race.Acquire(unsafe.Pointer(&rw.readerSem))
                        }
                        return true
                }
        }
}

// RUnlock undoes a single RLock call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func (rw *RWMutex) RUnlock() {
        if race.Enabled {
                _ = rw.w.state
                race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
                race.Disable()
        }
        if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
                // Outlined slow-path to allow the fast-path to be inlined
                rw.rUnlockSlow(r)
        }
        if race.Enabled {
                race.Enable()
        }
}

func (rw *RWMutex) rUnlockSlow(r int32) {
        if r+1 == 0 || r+1 == -rwmutexMaxReaders {
                race.Enable()
                throw("sync: RUnlock of unlocked RWMutex")
        }
        // A writer is pending.
        if atomic.AddInt32(&rw.readerWait, -1) == 0 {
                // The last reader unblocks the writer.
                runtime_Semrelease(&rw.writerSem, false, 1)
        }
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {
        if race.Enabled {
                _ = rw.w.state
                race.Disable()
        }
        // First, resolve competition with other writers.
        rw.w.Lock()
        // Announce to readers there is a pending writer.
        r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
        // Wait for active readers.
        if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
                runtime_SemacquireMutex(&rw.writerSem, false, 0)
        }
        if race.Enabled {
                race.Enable()
                race.Acquire(unsafe.Pointer(&rw.readerSem))
                race.Acquire(unsafe.Pointer(&rw.writerSem))
        }
}

// TryLock tries to lock rw for writing and reports whether it succeeded.
//
// Note that while correct uses of TryLock do exist, they are rare,
// and use of TryLock is often a sign of a deeper problem
// in a particular use of mutexes.
func (rw *RWMutex) TryLock() bool {
        if race.Enabled {
                _ = rw.w.state
                race.Disable()
        }
        if !rw.w.TryLock() {
                if race.Enabled {
                        race.Enable()
                }
                return false
        }
        if !atomic.CompareAndSwapInt32(&rw.readerCount, 0, -rwmutexMaxReaders) {
                rw.w.Unlock()
                if race.Enabled {
                        race.Enable()
                }
                return false
        }
        if race.Enabled {
                race.Enable()
                race.Acquire(unsafe.Pointer(&rw.readerSem))
                race.Acquire(unsafe.Pointer(&rw.writerSem))
        }
        return true
}

// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked RWMutex is not associated with a particular
// goroutine. One goroutine may RLock (Lock) a RWMutex and then
// arrange for another goroutine to RUnlock (Unlock) it.
func (rw *RWMutex) Unlock() {
        if race.Enabled {
                _ = rw.w.state
                race.Release(unsafe.Pointer(&rw.readerSem))
                race.Disable()
        }

        // Announce to readers there is no active writer.
        r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
        if r >= rwmutexMaxReaders {
                race.Enable()
                throw("sync: Unlock of unlocked RWMutex")
        }
        // Unblock blocked readers, if any.
        for i := 0; i < int(r); i++ {
                runtime_Semrelease(&rw.readerSem, false, 0)
        }
        // Allow other writers to proceed.
        rw.w.Unlock()
        if race.Enabled {
                race.Enable()
        }
}

// RLocker returns a Locker interface that implements
// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
func (rw *RWMutex) RLocker() Locker {
        return (*rlocker)(rw)
}

type rlocker RWMutex

func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }