144 lines
4.0 KiB
Go
144 lines
4.0 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Copyright 2019 The gVisor Authors.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build go1.12
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// +build !go1.15
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// Check go:linkname function signatures when updating Go version.
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// This is mostly copied from the standard library's sync/rwmutex.go.
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//
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// Happens-before relationships indicated to the race detector:
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// - Unlock -> Lock (via writerSem)
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// - Unlock -> RLock (via readerSem)
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// - RUnlock -> Lock (via writerSem)
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// - DowngradeLock -> RLock (via readerSem)
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package gvsync
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import (
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"sync"
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"sync/atomic"
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"unsafe"
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)
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//go:linkname runtimeSemacquire sync.runtime_Semacquire
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func runtimeSemacquire(s *uint32)
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// DowngradableRWMutex is identical to sync.RWMutex, but adds the DowngradeLock
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// method.
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type DowngradableRWMutex struct {
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w sync.Mutex // held if there are pending writers
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writerSem uint32 // semaphore for writers to wait for completing readers
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readerSem uint32 // semaphore for readers to wait for completing writers
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readerCount int32 // number of pending readers
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readerWait int32 // number of departing readers
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}
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const rwmutexMaxReaders = 1 << 30
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// RLock locks rw for reading.
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func (rw *DowngradableRWMutex) RLock() {
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if RaceEnabled {
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RaceDisable()
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}
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if atomic.AddInt32(&rw.readerCount, 1) < 0 {
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// A writer is pending, wait for it.
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runtimeSemacquire(&rw.readerSem)
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}
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if RaceEnabled {
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RaceEnable()
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RaceAcquire(unsafe.Pointer(&rw.readerSem))
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}
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}
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// RUnlock undoes a single RLock call.
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func (rw *DowngradableRWMutex) RUnlock() {
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if RaceEnabled {
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RaceReleaseMerge(unsafe.Pointer(&rw.writerSem))
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RaceDisable()
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}
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if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
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if r+1 == 0 || r+1 == -rwmutexMaxReaders {
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panic("RUnlock of unlocked DowngradableRWMutex")
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}
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// A writer is pending.
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if atomic.AddInt32(&rw.readerWait, -1) == 0 {
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// The last reader unblocks the writer.
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runtimeSemrelease(&rw.writerSem, false, 0)
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}
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}
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if RaceEnabled {
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RaceEnable()
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}
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}
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// Lock locks rw for writing.
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func (rw *DowngradableRWMutex) Lock() {
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if RaceEnabled {
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RaceDisable()
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}
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// First, resolve competition with other writers.
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rw.w.Lock()
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// Announce to readers there is a pending writer.
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r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
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// Wait for active readers.
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if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
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runtimeSemacquire(&rw.writerSem)
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}
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if RaceEnabled {
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RaceEnable()
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RaceAcquire(unsafe.Pointer(&rw.writerSem))
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}
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}
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// Unlock unlocks rw for writing.
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func (rw *DowngradableRWMutex) Unlock() {
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if RaceEnabled {
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RaceRelease(unsafe.Pointer(&rw.writerSem))
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RaceRelease(unsafe.Pointer(&rw.readerSem))
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RaceDisable()
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}
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// Announce to readers there is no active writer.
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r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
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if r >= rwmutexMaxReaders {
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panic("Unlock of unlocked DowngradableRWMutex")
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}
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// Unblock blocked readers, if any.
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for i := 0; i < int(r); i++ {
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runtimeSemrelease(&rw.readerSem, false, 0)
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}
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// Allow other writers to proceed.
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rw.w.Unlock()
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if RaceEnabled {
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RaceEnable()
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}
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}
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// DowngradeLock atomically unlocks rw for writing and locks it for reading.
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func (rw *DowngradableRWMutex) DowngradeLock() {
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if RaceEnabled {
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RaceRelease(unsafe.Pointer(&rw.readerSem))
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RaceDisable()
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}
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// Announce to readers there is no active writer and one additional reader.
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r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders+1)
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if r >= rwmutexMaxReaders+1 {
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panic("DowngradeLock of unlocked DowngradableRWMutex")
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}
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// Unblock blocked readers, if any. Note that this loop starts as 1 since r
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// includes this goroutine.
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for i := 1; i < int(r); i++ {
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runtimeSemrelease(&rw.readerSem, false, 0)
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}
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// Allow other writers to proceed to rw.w.Lock(). Note that they will still
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// block on rw.writerSem since at least this reader exists, such that
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// DowngradeLock() is atomic with the previous write lock.
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rw.w.Unlock()
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if RaceEnabled {
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RaceEnable()
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}
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}
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