diff options
Diffstat (limited to 'pkg/gate/gate.go')
| -rw-r--r-- | pkg/gate/gate.go | 134 |
1 files changed, 0 insertions, 134 deletions
diff --git a/pkg/gate/gate.go b/pkg/gate/gate.go deleted file mode 100644 index bda6aae09..000000000 --- a/pkg/gate/gate.go +++ /dev/null @@ -1,134 +0,0 @@ -// 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 gate provides a usage Gate synchronization primitive. -package gate - -import ( - "sync/atomic" -) - -const ( - // gateClosed is the bit set in the gate's user count to indicate that - // it has been closed. It is the MSB of the 32-bit field; the other 31 - // bits carry the actual count. - gateClosed = 0x80000000 -) - -// 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. -// -// Many goroutines are allowed to enter the gate concurrently, but only one is -// allowed to close it. -// -// This is similar to a r/w critical section, except that goroutines "entering" -// never block: they either enter immediately or fail to enter. The closer will -// block waiting for all goroutines currently inside the gate to leave. -// -// This function is implemented efficiently. On x86, only one interlocked -// operation is performed on enter, and one on leave. -// -// 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 gates, 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 uint32 - done chan struct{} -} - -// 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 g == nil { - return false - } - - for { - v := atomic.LoadUint32(&g.userCount) - if v&gateClosed != 0 { - return false - } - - if atomic.CompareAndSwapUint32(&g.userCount, v, v+1) { - return true - } - } -} - -// 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() { - for { - v := atomic.LoadUint32(&g.userCount) - if v&^gateClosed == 0 { - panic("leaving a gate with zero usage count") - } - - if atomic.CompareAndSwapUint32(&g.userCount, v, v-1) { - if v == gateClosed+1 { - close(g.done) - } - return - } - } -} - -// Close closes the gate for entering, 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() { - for { - v := atomic.LoadUint32(&g.userCount) - if v&^gateClosed != 0 && g.done == nil { - g.done = make(chan struct{}) - } - if atomic.CompareAndSwapUint32(&g.userCount, v, v|gateClosed) { - if v&^gateClosed != 0 { - <-g.done - } - return - } - } -} |