github.com/SagerNet/gvisor@v0.0.0-20210707092255-7731c139d75c/pkg/amutex/amutex.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package amutex provides the implementation of an abortable mutex. It allows
    16  // the Lock() function to be canceled while it waits to acquire the mutex.
    17  package amutex
    18  
    19  import (
    20  	"sync/atomic"
    21  
    22  	"github.com/SagerNet/gvisor/pkg/context"
    23  	"github.com/SagerNet/gvisor/pkg/syserror"
    24  )
    25  
    26  // Sleeper must be implemented by users of the abortable mutex to allow for
    27  // cancellation of waits.
    28  type Sleeper = context.ChannelSleeper
    29  
    30  // NoopSleeper is a stateless no-op implementation of Sleeper for anonymous
    31  // embedding in other types that do not support cancelation.
    32  type NoopSleeper = context.Context
    33  
    34  // Block blocks until either receiving from ch succeeds (in which case it
    35  // returns nil) or sleeper is interrupted (in which case it returns
    36  // syserror.ErrInterrupted).
    37  func Block(sleeper Sleeper, ch <-chan struct{}) error {
    38  	cancel := sleeper.SleepStart()
    39  	select {
    40  	case <-ch:
    41  		sleeper.SleepFinish(true)
    42  		return nil
    43  	case <-cancel:
    44  		sleeper.SleepFinish(false)
    45  		return syserror.ErrInterrupted
    46  	}
    47  }
    48  
    49  // AbortableMutex is an abortable mutex. It allows Lock() to be aborted while it
    50  // waits to acquire the mutex.
    51  type AbortableMutex struct {
    52  	v  int32
    53  	ch chan struct{}
    54  }
    55  
    56  // Init initializes the abortable mutex.
    57  func (m *AbortableMutex) Init() {
    58  	m.v = 1
    59  	m.ch = make(chan struct{}, 1)
    60  }
    61  
    62  // Lock attempts to acquire the mutex, returning true on success. If something
    63  // is written to the "c" while Lock waits, the wait is aborted and false is
    64  // returned instead.
    65  func (m *AbortableMutex) Lock(s Sleeper) bool {
    66  	// Uncontended case.
    67  	if atomic.AddInt32(&m.v, -1) == 0 {
    68  		return true
    69  	}
    70  
    71  	var c <-chan struct{}
    72  	if s != nil {
    73  		c = s.SleepStart()
    74  	}
    75  
    76  	for {
    77  		// Try to acquire the mutex again, at the same time making sure
    78  		// that m.v is negative, which indicates to the owner of the
    79  		// lock that it is contended, which ill force it to try to wake
    80  		// someone up when it releases the mutex.
    81  		if v := atomic.LoadInt32(&m.v); v >= 0 && atomic.SwapInt32(&m.v, -1) == 1 {
    82  			if s != nil {
    83  				s.SleepFinish(true)
    84  			}
    85  			return true
    86  		}
    87  
    88  		// Wait for the owner to wake us up before trying again, or for
    89  		// the wait to be aborted by the provided channel.
    90  		select {
    91  		case <-m.ch:
    92  		case <-c:
    93  			// s must be non-nil, otherwise c would be nil and we'd
    94  			// never reach this path.
    95  			s.SleepFinish(false)
    96  			return false
    97  		}
    98  	}
    99  }
   100  
   101  // Unlock releases the mutex.
   102  func (m *AbortableMutex) Unlock() {
   103  	if atomic.SwapInt32(&m.v, 1) == 0 {
   104  		// There were no pending waiters.
   105  		return
   106  	}
   107  
   108  	// Wake some waiter up.
   109  	select {
   110  	case m.ch <- struct{}{}:
   111  	default:
   112  	}
   113  }