104 lines
3.4 KiB
Go
104 lines
3.4 KiB
Go
// Copyright 2020 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package syncevent
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import (
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"sync/atomic"
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"gvisor.dev/gvisor/pkg/atomicbitops"
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)
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// Receiver is an event sink that holds pending events and invokes a callback
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// whenever new events become pending. Receiver's methods may be called
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// concurrently from multiple goroutines.
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//
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// Receiver.Init() must be called before first use.
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type Receiver struct {
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// pending is the set of pending events. pending is accessed using atomic
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// memory operations.
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pending uint64
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// cb is notified when new events become pending. cb is immutable after
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// Init().
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cb ReceiverCallback
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}
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// ReceiverCallback receives callbacks from a Receiver.
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type ReceiverCallback interface {
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// NotifyPending is called when the corresponding Receiver has new pending
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// events.
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//
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// NotifyPending is called synchronously from Receiver.Notify(), so
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// implementations must not take locks that may be held by callers of
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// Receiver.Notify(). NotifyPending may be called concurrently from
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// multiple goroutines.
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NotifyPending()
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}
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// Init must be called before first use of r.
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func (r *Receiver) Init(cb ReceiverCallback) {
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r.cb = cb
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}
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// Pending returns the set of pending events.
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func (r *Receiver) Pending() Set {
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return Set(atomic.LoadUint64(&r.pending))
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}
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// Notify sets the given events as pending.
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func (r *Receiver) Notify(es Set) {
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p := Set(atomic.LoadUint64(&r.pending))
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// Optimization: Skip the atomic CAS on r.pending if all events are
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// already pending.
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if p&es == es {
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return
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}
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// When this is uncontended (the common case), CAS is faster than
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// atomic-OR because the former is inlined and the latter (which we
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// implement in assembly ourselves) is not.
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if !atomic.CompareAndSwapUint64(&r.pending, uint64(p), uint64(p|es)) {
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// If the CAS fails, fall back to atomic-OR.
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atomicbitops.OrUint64(&r.pending, uint64(es))
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}
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r.cb.NotifyPending()
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}
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// Ack unsets the given events as pending.
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func (r *Receiver) Ack(es Set) {
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p := Set(atomic.LoadUint64(&r.pending))
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// Optimization: Skip the atomic CAS on r.pending if all events are
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// already not pending.
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if p&es == 0 {
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return
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}
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// When this is uncontended (the common case), CAS is faster than
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// atomic-AND because the former is inlined and the latter (which we
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// implement in assembly ourselves) is not.
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if !atomic.CompareAndSwapUint64(&r.pending, uint64(p), uint64(p&^es)) {
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// If the CAS fails, fall back to atomic-AND.
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atomicbitops.AndUint64(&r.pending, ^uint64(es))
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}
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}
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// PendingAndAckAll unsets all events as pending and returns the set of
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// previously-pending events.
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//
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// PendingAndAckAll should only be used in preference to a call to Pending
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// followed by a conditional call to Ack when the caller expects events to be
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// pending (e.g. after a call to ReceiverCallback.NotifyPending()).
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func (r *Receiver) PendingAndAckAll() Set {
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return Set(atomic.SwapUint64(&r.pending, 0))
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}
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