diff options
Diffstat (limited to 'pkg/sync')
| -rw-r--r-- | pkg/sync/BUILD | 42 | ||||
| -rw-r--r-- | pkg/sync/downgradable_rwmutex_test.go | 149 | ||||
| -rw-r--r-- | pkg/sync/downgradable_rwmutex_unsafe.go | 143 | ||||
| -rw-r--r-- | pkg/sync/memmove_unsafe.go | 32 | ||||
| -rw-r--r-- | pkg/sync/norace_unsafe.go | 44 | ||||
| -rw-r--r-- | pkg/sync/race_unsafe.go | 50 | ||||
| -rw-r--r-- | pkg/sync/seqatomic_unsafe.go | 81 | ||||
| -rw-r--r-- | pkg/sync/seqatomictest/BUILD | 34 | ||||
| -rw-r--r-- | pkg/sync/seqatomictest/seqatomic_test.go | 132 | ||||
| -rw-r--r-- | pkg/sync/seqcount.go | 158 | ||||
| -rw-r--r-- | pkg/sync/seqcount_test.go | 162 | ||||
| -rw-r--r-- | pkg/sync/sync.go | 16 |
12 files changed, 1043 insertions, 0 deletions
diff --git a/pkg/sync/BUILD b/pkg/sync/BUILD new file mode 100644 index 000000000..1fc0c25b5 --- /dev/null +++ b/pkg/sync/BUILD @@ -0,0 +1,42 @@ +load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test") + +package( + default_visibility = ["//:sandbox"], + licenses = ["notice"], # Apache 2.0, portions BSD +) + +load("//tools/go_generics:defs.bzl", "go_template") + +go_template( + name = "generic_seqatomic", + srcs = ["seqatomic_unsafe.go"], + types = [ + "Value", + ], + deps = [ + ":sync", + ], +) + +go_library( + name = "sync", + srcs = [ + "downgradable_rwmutex_unsafe.go", + "memmove_unsafe.go", + "norace_unsafe.go", + "race_unsafe.go", + "seqcount.go", + "sync.go", + ], + importpath = "gvisor.googlesource.com/gvisor/pkg/sync", +) + +go_test( + name = "sync_test", + size = "small", + srcs = [ + "downgradable_rwmutex_test.go", + "seqcount_test.go", + ], + embed = [":sync"], +) diff --git a/pkg/sync/downgradable_rwmutex_test.go b/pkg/sync/downgradable_rwmutex_test.go new file mode 100644 index 000000000..99c1c8be0 --- /dev/null +++ b/pkg/sync/downgradable_rwmutex_test.go @@ -0,0 +1,149 @@ +// 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. + +// GOMAXPROCS=10 go test + +// Copy/pasted from the standard library's sync/rwmutex_test.go, except for the +// addition of downgradingWriter and the renaming of num_iterations to +// numIterations to shut up Golint. + +package sync + +import ( + "fmt" + "runtime" + "sync/atomic" + "testing" +) + +func parallelReader(m *DowngradableRWMutex, clocked, cunlock, cdone chan bool) { + m.RLock() + clocked <- true + <-cunlock + m.RUnlock() + cdone <- true +} + +func doTestParallelReaders(numReaders, gomaxprocs int) { + runtime.GOMAXPROCS(gomaxprocs) + var m DowngradableRWMutex + clocked := make(chan bool) + cunlock := make(chan bool) + cdone := make(chan bool) + for i := 0; i < numReaders; i++ { + go parallelReader(&m, clocked, cunlock, cdone) + } + // Wait for all parallel RLock()s to succeed. + for i := 0; i < numReaders; i++ { + <-clocked + } + for i := 0; i < numReaders; i++ { + cunlock <- true + } + // Wait for the goroutines to finish. + for i := 0; i < numReaders; i++ { + <-cdone + } +} + +func TestParallelReaders(t *testing.T) { + defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(-1)) + doTestParallelReaders(1, 4) + doTestParallelReaders(3, 4) + doTestParallelReaders(4, 2) +} + +func reader(rwm *DowngradableRWMutex, numIterations int, activity *int32, cdone chan bool) { + for i := 0; i < numIterations; i++ { + rwm.RLock() + n := atomic.AddInt32(activity, 1) + if n < 1 || n >= 10000 { + panic(fmt.Sprintf("wlock(%d)\n", n)) + } + for i := 0; i < 100; i++ { + } + atomic.AddInt32(activity, -1) + rwm.RUnlock() + } + cdone <- true +} + +func writer(rwm *DowngradableRWMutex, numIterations int, activity *int32, cdone chan bool) { + for i := 0; i < numIterations; i++ { + rwm.Lock() + n := atomic.AddInt32(activity, 10000) + if n != 10000 { + panic(fmt.Sprintf("wlock(%d)\n", n)) + } + for i := 0; i < 100; i++ { + } + atomic.AddInt32(activity, -10000) + rwm.Unlock() + } + cdone <- true +} + +func downgradingWriter(rwm *DowngradableRWMutex, numIterations int, activity *int32, cdone chan bool) { + for i := 0; i < numIterations; i++ { + rwm.Lock() + n := atomic.AddInt32(activity, 10000) + if n != 10000 { + panic(fmt.Sprintf("wlock(%d)\n", n)) + } + for i := 0; i < 100; i++ { + } + atomic.AddInt32(activity, -10000) + rwm.DowngradeLock() + n = atomic.AddInt32(activity, 1) + if n < 1 || n >= 10000 { + panic(fmt.Sprintf("wlock(%d)\n", n)) + } + for i := 0; i < 100; i++ { + } + n = atomic.AddInt32(activity, -1) + rwm.RUnlock() + } + cdone <- true +} + +func HammerDowngradableRWMutex(gomaxprocs, numReaders, numIterations int) { + runtime.GOMAXPROCS(gomaxprocs) + // Number of active readers + 10000 * number of active writers. + var activity int32 + var rwm DowngradableRWMutex + cdone := make(chan bool) + go writer(&rwm, numIterations, &activity, cdone) + go downgradingWriter(&rwm, numIterations, &activity, cdone) + var i int + for i = 0; i < numReaders/2; i++ { + go reader(&rwm, numIterations, &activity, cdone) + } + go writer(&rwm, numIterations, &activity, cdone) + go downgradingWriter(&rwm, numIterations, &activity, cdone) + for ; i < numReaders; i++ { + go reader(&rwm, numIterations, &activity, cdone) + } + // Wait for the 4 writers and all readers to finish. + for i := 0; i < 4+numReaders; i++ { + <-cdone + } +} + +func TestDowngradableRWMutex(t *testing.T) { + defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(-1)) + n := 1000 + if testing.Short() { + n = 5 + } + HammerDowngradableRWMutex(1, 1, n) + HammerDowngradableRWMutex(1, 3, n) + HammerDowngradableRWMutex(1, 10, n) + HammerDowngradableRWMutex(4, 1, n) + HammerDowngradableRWMutex(4, 3, n) + HammerDowngradableRWMutex(4, 10, n) + HammerDowngradableRWMutex(10, 1, n) + HammerDowngradableRWMutex(10, 3, n) + HammerDowngradableRWMutex(10, 10, n) + HammerDowngradableRWMutex(10, 5, n) +} diff --git a/pkg/sync/downgradable_rwmutex_unsafe.go b/pkg/sync/downgradable_rwmutex_unsafe.go new file mode 100644 index 000000000..9a96056fc --- /dev/null +++ b/pkg/sync/downgradable_rwmutex_unsafe.go @@ -0,0 +1,143 @@ +// 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. + +// This is mostly copied from the standard library's sync/rwmutex.go. +// +// Happens-before relationships indicated to the race detector: +// - Unlock -> Lock (via writerSem) +// - Unlock -> RLock (via readerSem) +// - RUnlock -> Lock (via writerSem) +// - DowngradeLock -> RLock (via readerSem) + +package sync + +import ( + "sync" + "sync/atomic" + "unsafe" +) + +// DowngradableRWMutex is identical to sync.RWMutex, but adds the DowngradeLock +// method. +type DowngradableRWMutex struct { + w sync.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 + +// RLock locks rw for reading. +func (rw *DowngradableRWMutex) RLock() { + if RaceEnabled { + RaceDisable() + } + if atomic.AddInt32(&rw.readerCount, 1) < 0 { + // A writer is pending, wait for it. + runtimeSemacquire(&rw.readerSem) + } + if RaceEnabled { + RaceEnable() + RaceAcquire(unsafe.Pointer(&rw.readerSem)) + } +} + +// RUnlock undoes a single RLock call. +func (rw *DowngradableRWMutex) RUnlock() { + if RaceEnabled { + // TODO: Why does this need to be ReleaseMerge instead of + // Release? IIUC this establishes Unlock happens-before RUnlock, which + // seems unnecessary. + RaceReleaseMerge(unsafe.Pointer(&rw.writerSem)) + RaceDisable() + } + if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 { + if r+1 == 0 || r+1 == -rwmutexMaxReaders { + panic("RUnlock of unlocked DowngradableRWMutex") + } + // A writer is pending. + if atomic.AddInt32(&rw.readerWait, -1) == 0 { + // The last reader unblocks the writer. + runtimeSemrelease(&rw.writerSem, false) + } + } + if RaceEnabled { + RaceEnable() + } +} + +// Lock locks rw for writing. +func (rw *DowngradableRWMutex) Lock() { + if RaceEnabled { + RaceDisable() + } + // 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 { + runtimeSemacquire(&rw.writerSem) + } + if RaceEnabled { + RaceEnable() + RaceAcquire(unsafe.Pointer(&rw.writerSem)) + } +} + +// Unlock unlocks rw for writing. +func (rw *DowngradableRWMutex) Unlock() { + if RaceEnabled { + RaceRelease(unsafe.Pointer(&rw.writerSem)) + RaceRelease(unsafe.Pointer(&rw.readerSem)) + RaceDisable() + } + // Announce to readers there is no active writer. + r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders) + if r >= rwmutexMaxReaders { + panic("Unlock of unlocked DowngradableRWMutex") + } + // Unblock blocked readers, if any. + for i := 0; i < int(r); i++ { + runtimeSemrelease(&rw.readerSem, false) + } + // Allow other writers to proceed. + rw.w.Unlock() + if RaceEnabled { + RaceEnable() + } +} + +// DowngradeLock atomically unlocks rw for writing and locks it for reading. +func (rw *DowngradableRWMutex) DowngradeLock() { + if RaceEnabled { + RaceRelease(unsafe.Pointer(&rw.readerSem)) + RaceDisable() + } + // Announce to readers there is no active writer and one additional reader. + r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders+1) + if r >= rwmutexMaxReaders+1 { + panic("DowngradeLock of unlocked DowngradableRWMutex") + } + // Unblock blocked readers, if any. Note that this loop starts as 1 since r + // includes this goroutine. + for i := 1; i < int(r); i++ { + runtimeSemrelease(&rw.readerSem, false) + } + // Allow other writers to proceed to rw.w.Lock(). Note that they will still + // block on rw.writerSem since at least this reader exists, such that + // DowngradeLock() is atomic with the previous write lock. + rw.w.Unlock() + if RaceEnabled { + RaceEnable() + } +} + +//go:linkname runtimeSemacquire sync.runtime_Semacquire +func runtimeSemacquire(s *uint32) + +//go:linkname runtimeSemrelease sync.runtime_Semrelease +func runtimeSemrelease(s *uint32, handoff bool) diff --git a/pkg/sync/memmove_unsafe.go b/pkg/sync/memmove_unsafe.go new file mode 100644 index 000000000..0c992d5a4 --- /dev/null +++ b/pkg/sync/memmove_unsafe.go @@ -0,0 +1,32 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package sync + +import ( + "unsafe" +) + +// Memmove is exported for SeqAtomicLoad/SeqAtomicTryLoad<T>, which can't +// define it because go_generics can't update the go:linkname annotation. +// Furthermore, go:linkname silently doesn't work if the local name is exported +// (this is of course undocumented), which is why this indirection is +// necessary. +func Memmove(to, from unsafe.Pointer, n uintptr) { + memmove(to, from, n) +} + +//go:linkname memmove runtime.memmove +//go:noescape +func memmove(to, from unsafe.Pointer, n uintptr) diff --git a/pkg/sync/norace_unsafe.go b/pkg/sync/norace_unsafe.go new file mode 100644 index 000000000..968665078 --- /dev/null +++ b/pkg/sync/norace_unsafe.go @@ -0,0 +1,44 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +// +build !race + +package sync + +import ( + "unsafe" +) + +// RaceEnabled is true if the Go data race detector is enabled. +const RaceEnabled = false + +// RaceDisable has the same semantics as runtime.RaceDisable. +func RaceDisable() { +} + +// RaceEnable has the same semantics as runtime.RaceEnable. +func RaceEnable() { +} + +// RaceAcquire has the same semantics as runtime.RaceAcquire. +func RaceAcquire(addr unsafe.Pointer) { +} + +// RaceRelease has the same semantics as runtime.RaceRelease. +func RaceRelease(addr unsafe.Pointer) { +} + +// RaceReleaseMerge has the same semantics as runtime.RaceReleaseMerge. +func RaceReleaseMerge(addr unsafe.Pointer) { +} diff --git a/pkg/sync/race_unsafe.go b/pkg/sync/race_unsafe.go new file mode 100644 index 000000000..d143a21c7 --- /dev/null +++ b/pkg/sync/race_unsafe.go @@ -0,0 +1,50 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +// +build race + +package sync + +import ( + "runtime" + "unsafe" +) + +// RaceEnabled is true if the Go data race detector is enabled. +const RaceEnabled = true + +// RaceDisable has the same semantics as runtime.RaceDisable. +func RaceDisable() { + runtime.RaceDisable() +} + +// RaceEnable has the same semantics as runtime.RaceEnable. +func RaceEnable() { + runtime.RaceEnable() +} + +// RaceAcquire has the same semantics as runtime.RaceAcquire. +func RaceAcquire(addr unsafe.Pointer) { + runtime.RaceAcquire(addr) +} + +// RaceRelease has the same semantics as runtime.RaceRelease. +func RaceRelease(addr unsafe.Pointer) { + runtime.RaceRelease(addr) +} + +// RaceReleaseMerge has the same semantics as runtime.RaceReleaseMerge. +func RaceReleaseMerge(addr unsafe.Pointer) { + runtime.RaceReleaseMerge(addr) +} diff --git a/pkg/sync/seqatomic_unsafe.go b/pkg/sync/seqatomic_unsafe.go new file mode 100644 index 000000000..a18e1229a --- /dev/null +++ b/pkg/sync/seqatomic_unsafe.go @@ -0,0 +1,81 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +// Package template doesn't exist. This file must be instantiated using the +// go_template_instance rule in tools/go_generics/defs.bzl. +package template + +import ( + "fmt" + "reflect" + "strings" + "unsafe" + + ssync "gvisor.googlesource.com/gvisor/pkg/sync" +) + +// Value is a required type parameter. +// +// Value must not contain any pointers, including interface objects, function +// objects, slices, maps, channels, unsafe.Pointer, and arrays or structs +// containing any of the above. An init() function will panic if this property +// does not hold. +type Value struct{} + +// SeqAtomicLoad returns a copy of *ptr, ensuring that the read does not race +// with any writer critical sections in sc. +func SeqAtomicLoad(sc *ssync.SeqCount, ptr *Value) Value { + // This function doesn't use SeqAtomicTryLoad because doing so is + // measurably, significantly (~20%) slower; Go is awful at inlining. + var val Value + for { + epoch := sc.BeginRead() + if ssync.RaceEnabled { + // runtime.RaceDisable() doesn't actually stop the race detector, + // so it can't help us here. Instead, call runtime.memmove + // directly, which is not instrumented by the race detector. + ssync.Memmove(unsafe.Pointer(&val), unsafe.Pointer(ptr), unsafe.Sizeof(val)) + } else { + // This is ~40% faster for short reads than going through memmove. + val = *ptr + } + if sc.ReadOk(epoch) { + break + } + } + return val +} + +// SeqAtomicTryLoad returns a copy of *ptr while in a reader critical section +// in sc initiated by a call to sc.BeginRead() that returned epoch. If the read +// would race with a writer critical section, SeqAtomicTryLoad returns +// (unspecified, false). +func SeqAtomicTryLoad(sc *ssync.SeqCount, epoch ssync.SeqCountEpoch, ptr *Value) (Value, bool) { + var val Value + if ssync.RaceEnabled { + ssync.Memmove(unsafe.Pointer(&val), unsafe.Pointer(ptr), unsafe.Sizeof(val)) + } else { + val = *ptr + } + return val, sc.ReadOk(epoch) +} + +func init() { + var val Value + typ := reflect.TypeOf(val) + name := typ.Name() + if ptrs := ssync.PointersInType(typ, name); len(ptrs) != 0 { + panic(fmt.Sprintf("SeqAtomicLoad<%s> is invalid since values %s of type %s contain pointers:\n%s", typ, name, typ, strings.Join(ptrs, "\n"))) + } +} diff --git a/pkg/sync/seqatomictest/BUILD b/pkg/sync/seqatomictest/BUILD new file mode 100644 index 000000000..9d6ee2dfb --- /dev/null +++ b/pkg/sync/seqatomictest/BUILD @@ -0,0 +1,34 @@ +package(licenses = ["notice"]) # Apache 2.0 + +load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test") +load("//tools/go_generics:defs.bzl", "go_template_instance") + +go_template_instance( + name = "seqatomic_int", + out = "seqatomic_int.go", + package = "seqatomic", + suffix = "Int", + template = "//pkg/sync:generic_seqatomic", + types = { + "Value": "int", + }, +) + +go_library( + name = "seqatomic", + srcs = ["seqatomic_int.go"], + importpath = "gvisor.googlesource.com/gvisor/pkg/sync/seqatomic", + deps = [ + "//pkg/sync", + ], +) + +go_test( + name = "seqatomic_test", + size = "small", + srcs = ["seqatomic_test.go"], + embed = [":seqatomic"], + deps = [ + "//pkg/sync", + ], +) diff --git a/pkg/sync/seqatomictest/seqatomic_test.go b/pkg/sync/seqatomictest/seqatomic_test.go new file mode 100644 index 000000000..b785d2344 --- /dev/null +++ b/pkg/sync/seqatomictest/seqatomic_test.go @@ -0,0 +1,132 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package seqatomic + +import ( + "sync/atomic" + "testing" + "time" + + gvsync "gvisor.googlesource.com/gvisor/pkg/sync" +) + +func TestSeqAtomicLoadUncontended(t *testing.T) { + var seq gvsync.SeqCount + const want = 1 + data := want + if got := SeqAtomicLoadInt(&seq, &data); got != want { + t.Errorf("SeqAtomicLoadInt: got %v, wanted %v", got, want) + } +} + +func TestSeqAtomicLoadAfterWrite(t *testing.T) { + var seq gvsync.SeqCount + var data int + const want = 1 + seq.BeginWrite() + data = want + seq.EndWrite() + if got := SeqAtomicLoadInt(&seq, &data); got != want { + t.Errorf("SeqAtomicLoadInt: got %v, wanted %v", got, want) + } +} + +func TestSeqAtomicLoadDuringWrite(t *testing.T) { + var seq gvsync.SeqCount + var data int + const want = 1 + seq.BeginWrite() + go func() { + time.Sleep(time.Second) + data = want + seq.EndWrite() + }() + if got := SeqAtomicLoadInt(&seq, &data); got != want { + t.Errorf("SeqAtomicLoadInt: got %v, wanted %v", got, want) + } +} + +func TestSeqAtomicTryLoadUncontended(t *testing.T) { + var seq gvsync.SeqCount + const want = 1 + data := want + epoch := seq.BeginRead() + if got, ok := SeqAtomicTryLoadInt(&seq, epoch, &data); !ok || got != want { + t.Errorf("SeqAtomicTryLoadInt: got (%v, %v), wanted (%v, true)", got, ok, want) + } +} + +func TestSeqAtomicTryLoadDuringWrite(t *testing.T) { + var seq gvsync.SeqCount + var data int + epoch := seq.BeginRead() + seq.BeginWrite() + if got, ok := SeqAtomicTryLoadInt(&seq, epoch, &data); ok { + t.Errorf("SeqAtomicTryLoadInt: got (%v, true), wanted (_, false)", got) + } + seq.EndWrite() +} + +func TestSeqAtomicTryLoadAfterWrite(t *testing.T) { + var seq gvsync.SeqCount + var data int + epoch := seq.BeginRead() + seq.BeginWrite() + seq.EndWrite() + if got, ok := SeqAtomicTryLoadInt(&seq, epoch, &data); ok { + t.Errorf("SeqAtomicTryLoadInt: got (%v, true), wanted (_, false)", got) + } +} + +func BenchmarkSeqAtomicLoadIntUncontended(b *testing.B) { + var seq gvsync.SeqCount + const want = 42 + data := want + b.RunParallel(func(pb *testing.PB) { + for pb.Next() { + if got := SeqAtomicLoadInt(&seq, &data); got != want { + b.Fatalf("SeqAtomicLoadInt: got %v, wanted %v", got, want) + } + } + }) +} + +func BenchmarkSeqAtomicTryLoadIntUncontended(b *testing.B) { + var seq gvsync.SeqCount + const want = 42 + data := want + b.RunParallel(func(pb *testing.PB) { + epoch := seq.BeginRead() + for pb.Next() { + if got, ok := SeqAtomicTryLoadInt(&seq, epoch, &data); !ok || got != want { + b.Fatalf("SeqAtomicTryLoadInt: got (%v, %v), wanted (%v, true)", got, ok, want) + } + } + }) +} + +// For comparison: +func BenchmarkAtomicValueLoadIntUncontended(b *testing.B) { + var a atomic.Value + const want = 42 + a.Store(int(want)) + b.RunParallel(func(pb *testing.PB) { + for pb.Next() { + if got := a.Load().(int); got != want { + b.Fatalf("atomic.Value.Load: got %v, wanted %v", got, want) + } + } + }) +} diff --git a/pkg/sync/seqcount.go b/pkg/sync/seqcount.go new file mode 100644 index 000000000..8e3304d69 --- /dev/null +++ b/pkg/sync/seqcount.go @@ -0,0 +1,158 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package sync + +import ( + "fmt" + "reflect" + "runtime" + "sync/atomic" +) + +// SeqCount is a synchronization primitive for optimistic reader/writer +// synchronization in cases where readers can work with stale data and +// therefore do not need to block writers. +// +// Compared to sync/atomic.Value: +// +// - Mutation of SeqCount-protected data does not require memory allocation, +// whereas atomic.Value generally does. This is a significant advantage when +// writes are common. +// +// - Atomic reads of SeqCount-protected data require copying. This is a +// disadvantage when atomic reads are common. +// +// - SeqCount may be more flexible: correct use of SeqCount.ReadOk allows other +// operations to be made atomic with reads of SeqCount-protected data. +// +// - SeqCount may be less flexible: as of this writing, SeqCount-protected data +// cannot include pointers. +// +// - SeqCount is more cumbersome to use; atomic reads of SeqCount-protected +// data require instantiating function templates using go_generics (see +// seqatomic.go). +type SeqCount struct { + // epoch is incremented by BeginWrite and EndWrite, such that epoch is odd + // if a writer critical section is active, and a read from data protected + // by this SeqCount is atomic iff epoch is the same even value before and + // after the read. + epoch uint32 +} + +// SeqCountEpoch tracks writer critical sections in a SeqCount. +type SeqCountEpoch struct { + val uint32 +} + +// We assume that: +// +// - All functions in sync/atomic that perform a memory read are at least a +// read fence: memory reads before calls to such functions cannot be reordered +// after the call, and memory reads after calls to such functions cannot be +// reordered before the call, even if those reads do not use sync/atomic. +// +// - All functions in sync/atomic that perform a memory write are at least a +// write fence: memory writes before calls to such functions cannot be +// reordered after the call, and memory writes after calls to such functions +// cannot be reordered before the call, even if those writes do not use +// sync/atomic. +// +// As of this writing, the Go memory model completely fails to describe +// sync/atomic, but these properties are implied by +// https://groups.google.com/forum/#!topic/golang-nuts/7EnEhM3U7B8. + +// BeginRead indicates the beginning of a reader critical section. Reader +// critical sections DO NOT BLOCK writer critical sections, so operations in a +// reader critical section MAY RACE with writer critical sections. Races are +// detected by ReadOk at the end of the reader critical section. Thus, the +// low-level structure of readers is generally: +// +// for { +// epoch := seq.BeginRead() +// // do something idempotent with seq-protected data +// if seq.ReadOk(epoch) { +// break +// } +// } +// +// However, since reader critical sections may race with writer critical +// sections, the Go race detector will (accurately) flag data races in readers +// using this pattern. Most users of SeqCount will need to use the +// SeqAtomicLoad function template in seqatomic.go. +func (s *SeqCount) BeginRead() SeqCountEpoch { + epoch := atomic.LoadUint32(&s.epoch) + for epoch&1 != 0 { + runtime.Gosched() + epoch = atomic.LoadUint32(&s.epoch) + } + return SeqCountEpoch{epoch} +} + +// ReadOk returns true if the reader critical section initiated by a previous +// call to BeginRead() that returned epoch did not race with any writer critical +// sections. +// +// ReadOk may be called any number of times during a reader critical section. +// Reader critical sections do not need to be explicitly terminated; the last +// call to ReadOk is implicitly the end of the reader critical section. +func (s *SeqCount) ReadOk(epoch SeqCountEpoch) bool { + return atomic.LoadUint32(&s.epoch) == epoch.val +} + +// BeginWrite indicates the beginning of a writer critical section. +// +// SeqCount does not support concurrent writer critical sections; clients with +// concurrent writers must synchronize them using e.g. sync.Mutex. +func (s *SeqCount) BeginWrite() { + if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 == 0 { + panic("SeqCount.BeginWrite during writer critical section") + } +} + +// EndWrite ends the effect of a preceding BeginWrite. +func (s *SeqCount) EndWrite() { + if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 != 0 { + panic("SeqCount.EndWrite outside writer critical section") + } +} + +// PointersInType returns a list of pointers reachable from values named +// valName of the given type. +// +// PointersInType is not exhaustive, but it is guaranteed that if typ contains +// at least one pointer, then PointersInTypeOf returns a non-empty list. +func PointersInType(typ reflect.Type, valName string) []string { + switch kind := typ.Kind(); kind { + case reflect.Bool, reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128: + return nil + + case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice, reflect.String, reflect.UnsafePointer: + return []string{valName} + + case reflect.Array: + return PointersInType(typ.Elem(), valName+"[]") + + case reflect.Struct: + var ptrs []string + for i, n := 0, typ.NumField(); i < n; i++ { + field := typ.Field(i) + ptrs = append(ptrs, PointersInType(field.Type, fmt.Sprintf("%s.%s", valName, field.Name))...) + } + return ptrs + + default: + return []string{fmt.Sprintf("%s (of type %s with unknown kind %s)", valName, typ, kind)} + } +} diff --git a/pkg/sync/seqcount_test.go b/pkg/sync/seqcount_test.go new file mode 100644 index 000000000..fa4abed1d --- /dev/null +++ b/pkg/sync/seqcount_test.go @@ -0,0 +1,162 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package sync + +import ( + "reflect" + "testing" + "time" +) + +func TestSeqCountWriteUncontended(t *testing.T) { + var seq SeqCount + seq.BeginWrite() + seq.EndWrite() +} + +func TestSeqCountReadUncontended(t *testing.T) { + var seq SeqCount + epoch := seq.BeginRead() + if !seq.ReadOk(epoch) { + t.Errorf("ReadOk: got false, wanted true") + } +} + +func TestSeqCountBeginReadAfterWrite(t *testing.T) { + var seq SeqCount + var data int32 + const want = 1 + seq.BeginWrite() + data = want + seq.EndWrite() + epoch := seq.BeginRead() + if data != want { + t.Errorf("Reader: got %v, wanted %v", data, want) + } + if !seq.ReadOk(epoch) { + t.Errorf("ReadOk: got false, wanted true") + } +} + +func TestSeqCountBeginReadDuringWrite(t *testing.T) { + var seq SeqCount + var data int + const want = 1 + seq.BeginWrite() + go func() { + time.Sleep(time.Second) + data = want + seq.EndWrite() + }() + epoch := seq.BeginRead() + if data != want { + t.Errorf("Reader: got %v, wanted %v", data, want) + } + if !seq.ReadOk(epoch) { + t.Errorf("ReadOk: got false, wanted true") + } +} + +func TestSeqCountReadOkAfterWrite(t *testing.T) { + var seq SeqCount + epoch := seq.BeginRead() + seq.BeginWrite() + seq.EndWrite() + if seq.ReadOk(epoch) { + t.Errorf("ReadOk: got true, wanted false") + } +} + +func TestSeqCountReadOkDuringWrite(t *testing.T) { + var seq SeqCount + epoch := seq.BeginRead() + seq.BeginWrite() + if seq.ReadOk(epoch) { + t.Errorf("ReadOk: got true, wanted false") + } + seq.EndWrite() +} + +func BenchmarkSeqCountWriteUncontended(b *testing.B) { + var seq SeqCount + for i := 0; i < b.N; i++ { + seq.BeginWrite() + seq.EndWrite() + } +} + +func BenchmarkSeqCountReadUncontended(b *testing.B) { + var seq SeqCount + b.RunParallel(func(pb *testing.PB) { + for pb.Next() { + epoch := seq.BeginRead() + if !seq.ReadOk(epoch) { + b.Fatalf("ReadOk: got false, wanted true") + } + } + }) +} + +func TestPointersInType(t *testing.T) { + for _, test := range []struct { + name string // used for both test and value name + val interface{} + ptrs []string + }{ + { + name: "EmptyStruct", + val: struct{}{}, + }, + { + name: "Int", + val: int(0), + }, + { + name: "MixedStruct", + val: struct { + b bool + I int + ExportedPtr *struct{} + unexportedPtr *struct{} + arr [2]int + ptrArr [2]*int + nestedStruct struct { + nestedNonptr int + nestedPtr *int + } + structArr [1]struct { + nonptr int + ptr *int + } + }{}, + ptrs: []string{ + "MixedStruct.ExportedPtr", + "MixedStruct.unexportedPtr", + "MixedStruct.ptrArr[]", + "MixedStruct.nestedStruct.nestedPtr", + "MixedStruct.structArr[].ptr", + }, + }, + } { + t.Run(test.name, func(t *testing.T) { + typ := reflect.TypeOf(test.val) + ptrs := PointersInType(typ, test.name) + t.Logf("Found pointers: %v", ptrs) + if (len(ptrs) != 0 || len(test.ptrs) != 0) && !reflect.DeepEqual(ptrs, test.ptrs) { + t.Errorf("Got %v, wanted %v", ptrs, test.ptrs) + } + }) + } +} diff --git a/pkg/sync/sync.go b/pkg/sync/sync.go new file mode 100644 index 000000000..36d4c4dee --- /dev/null +++ b/pkg/sync/sync.go @@ -0,0 +1,16 @@ +// Copyright 2018 Google Inc. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +// Package sync provides synchronization primitives. +package sync |