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Diffstat (limited to 'pkg/sync/gate_unsafe.go')
| -rw-r--r-- | pkg/sync/gate_unsafe.go | 150 |
1 files changed, 150 insertions, 0 deletions
diff --git a/pkg/sync/gate_unsafe.go b/pkg/sync/gate_unsafe.go new file mode 100644 index 000000000..ae32287ef --- /dev/null +++ b/pkg/sync/gate_unsafe.go @@ -0,0 +1,150 @@ +// Copyright 2018 The gVisor Authors. +// +// 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" + "math" + "sync/atomic" + "unsafe" + + "gvisor.dev/gvisor/pkg/gohacks" +) + +// Gate is a synchronization primitive that allows concurrent goroutines to +// "enter" it as long as it hasn't been closed yet. Once it's been closed, +// goroutines cannot enter it anymore, but are allowed to leave, and the closer +// will be informed when all goroutines have left. +// +// Gate is similar to WaitGroup: +// +// - Gate.Enter() is analogous to WaitGroup.Add(1), but may be called even if +// the Gate counter is 0 and fails if Gate.Close() has been called. +// +// - Gate.Leave() is equivalent to WaitGroup.Done(). +// +// - Gate.Close() is analogous to WaitGroup.Wait(), but also causes future +// calls to Gate.Enter() to fail and may only be called once, from a single +// goroutine. +// +// This is useful, for example, in cases when a goroutine is trying to clean up +// an object for which multiple goroutines have pointers. In such a case, users +// would be required to enter and leave the Gate, and the cleaner would wait +// until all users are gone (and no new ones are allowed) before proceeding. +// +// Users: +// +// if !g.Enter() { +// // Gate is closed, we can't use the object. +// return +// } +// +// // Do something with object. +// [...] +// +// g.Leave() +// +// Closer: +// +// // Prevent new users from using the object, and wait for the existing +// // ones to complete. +// g.Close() +// +// // Clean up the object. +// [...] +// +type Gate struct { + userCount int32 + closingG uintptr +} + +const preparingG = 1 + +// Enter tries to enter the gate. It will succeed if it hasn't been closed yet, +// in which case the caller must eventually call Leave(). +// +// This function is thread-safe. +func (g *Gate) Enter() bool { + if atomic.AddInt32(&g.userCount, 1) > 0 { + return true + } + g.leaveAfterFailedEnter() + return false +} + +// leaveAfterFailedEnter is identical to Leave, but is marked noinline to +// prevent it from being inlined into Enter, since as of this writing inlining +// Leave into Enter prevents Enter from being inlined into its callers. +//go:noinline +func (g *Gate) leaveAfterFailedEnter() { + if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 { + g.leaveClosed() + } +} + +// Leave leaves the gate. This must only be called after a successful call to +// Enter(). If the gate has been closed and this is the last one inside the +// gate, it will notify the closer that the gate is done. +// +// This function is thread-safe. +func (g *Gate) Leave() { + if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 { + g.leaveClosed() + } +} + +func (g *Gate) leaveClosed() { + if atomic.LoadUintptr(&g.closingG) == 0 { + return + } + if g := atomic.SwapUintptr(&g.closingG, 0); g > preparingG { + goready(g, 0) + } +} + +// Close closes the gate, causing future calls to Enter to fail, and waits +// until all goroutines that are currently inside the gate leave before +// returning. +// +// Only one goroutine can call this function. +func (g *Gate) Close() { + if atomic.LoadInt32(&g.userCount) == math.MinInt32 { + // The gate is already closed, with no goroutines inside. For legacy + // reasons, we have to allow Close to be called again in this case. + return + } + if v := atomic.AddInt32(&g.userCount, math.MinInt32); v == math.MinInt32 { + // userCount was already 0. + return + } else if v >= 0 { + panic("concurrent Close of sync.Gate") + } + + if g := atomic.SwapUintptr(&g.closingG, preparingG); g != 0 { + panic(fmt.Sprintf("invalid sync.Gate.closingG during Close: %#x", g)) + } + if atomic.LoadInt32(&g.userCount) == math.MinInt32 { + // The last call to Leave arrived while we were setting up closingG. + return + } + // WaitReasonSemacquire/TraceEvGoBlockSync are consistent with WaitGroup. + gopark(gateCommit, gohacks.Noescape(unsafe.Pointer(&g.closingG)), WaitReasonSemacquire, TraceEvGoBlockSync, 0) +} + +//go:norace +//go:nosplit +func gateCommit(g uintptr, closingG unsafe.Pointer) bool { + return RaceUncheckedAtomicCompareAndSwapUintptr((*uintptr)(closingG), preparingG, g) +} |