github.com/ttpreport/gvisor-ligolo@v0.0.0-20240123134145-a858404967ba/pkg/sync/gate_unsafe.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 sync
    16  
    17  import (
    18  	"fmt"
    19  	"math"
    20  	"sync/atomic"
    21  	"unsafe"
    22  
    23  	"github.com/ttpreport/gvisor-ligolo/pkg/gohacks"
    24  )
    25  
    26  // Gate is a synchronization primitive that allows concurrent goroutines to
    27  // "enter" it as long as it hasn't been closed yet. Once it's been closed,
    28  // goroutines cannot enter it anymore, but are allowed to leave, and the closer
    29  // will be informed when all goroutines have left.
    30  //
    31  // Gate is similar to WaitGroup:
    32  //
    33  //   - Gate.Enter() is analogous to WaitGroup.Add(1), but may be called even if
    34  //     the Gate counter is 0 and fails if Gate.Close() has been called.
    35  //
    36  //   - Gate.Leave() is equivalent to WaitGroup.Done().
    37  //
    38  //   - Gate.Close() is analogous to WaitGroup.Wait(), but also causes future
    39  //
    40  // calls to Gate.Enter() to fail and may only be called once, from a single
    41  // goroutine.
    42  //
    43  // This is useful, for example, in cases when a goroutine is trying to clean up
    44  // an object for which multiple goroutines have pointers. In such a case, users
    45  // would be required to enter and leave the Gate, and the cleaner would wait
    46  // until all users are gone (and no new ones are allowed) before proceeding.
    47  //
    48  // Users:
    49  //
    50  //	if !g.Enter() {
    51  //		// Gate is closed, we can't use the object.
    52  //		return
    53  //	}
    54  //
    55  //	// Do something with object.
    56  //	[...]
    57  //
    58  //	g.Leave()
    59  //
    60  // Closer:
    61  //
    62  //	// Prevent new users from using the object, and wait for the existing
    63  //	// ones to complete.
    64  //	g.Close()
    65  //
    66  //	// Clean up the object.
    67  //	[...]
    68  type Gate struct {
    69  	userCount int32
    70  	closingG  uintptr
    71  }
    72  
    73  const preparingG = 1
    74  
    75  // Enter tries to enter the gate. It will succeed if it hasn't been closed yet,
    76  // in which case the caller must eventually call Leave().
    77  //
    78  // This function is thread-safe.
    79  func (g *Gate) Enter() bool {
    80  	if atomic.AddInt32(&g.userCount, 1) > 0 {
    81  		return true
    82  	}
    83  	g.leaveAfterFailedEnter()
    84  	return false
    85  }
    86  
    87  // leaveAfterFailedEnter is identical to Leave, but is marked noinline to
    88  // prevent it from being inlined into Enter, since as of this writing inlining
    89  // Leave into Enter prevents Enter from being inlined into its callers.
    90  //
    91  //go:noinline
    92  func (g *Gate) leaveAfterFailedEnter() {
    93  	if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 {
    94  		g.leaveClosed()
    95  	}
    96  }
    97  
    98  // Leave leaves the gate. This must only be called after a successful call to
    99  // Enter(). If the gate has been closed and this is the last one inside the
   100  // gate, it will notify the closer that the gate is done.
   101  //
   102  // This function is thread-safe.
   103  func (g *Gate) Leave() {
   104  	if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 {
   105  		g.leaveClosed()
   106  	}
   107  }
   108  
   109  func (g *Gate) leaveClosed() {
   110  	if atomic.LoadUintptr(&g.closingG) == 0 {
   111  		return
   112  	}
   113  	if g := atomic.SwapUintptr(&g.closingG, 0); g > preparingG {
   114  		goready(g, 0)
   115  	}
   116  }
   117  
   118  // Close closes the gate, causing future calls to Enter to fail, and waits
   119  // until all goroutines that are currently inside the gate leave before
   120  // returning.
   121  //
   122  // Only one goroutine can call this function.
   123  func (g *Gate) Close() {
   124  	if atomic.LoadInt32(&g.userCount) == math.MinInt32 {
   125  		// The gate is already closed, with no goroutines inside. For legacy
   126  		// reasons, we have to allow Close to be called again in this case.
   127  		return
   128  	}
   129  	if v := atomic.AddInt32(&g.userCount, math.MinInt32); v == math.MinInt32 {
   130  		// userCount was already 0.
   131  		return
   132  	} else if v >= 0 {
   133  		panic("concurrent Close of sync.Gate")
   134  	}
   135  
   136  	if g := atomic.SwapUintptr(&g.closingG, preparingG); g != 0 {
   137  		panic(fmt.Sprintf("invalid sync.Gate.closingG during Close: %#x", g))
   138  	}
   139  	if atomic.LoadInt32(&g.userCount) == math.MinInt32 {
   140  		// The last call to Leave arrived while we were setting up closingG.
   141  		return
   142  	}
   143  	// WaitReasonSemacquire/TraceEvGoBlockSync are consistent with WaitGroup.
   144  	gopark(gateCommit, gohacks.Noescape(unsafe.Pointer(&g.closingG)), WaitReasonSemacquire, TraceEvGoBlockSync, 0)
   145  }
   146  
   147  //go:norace
   148  //go:nosplit
   149  func gateCommit(g uintptr, closingG unsafe.Pointer) bool {
   150  	return RaceUncheckedAtomicCompareAndSwapUintptr((*uintptr)(closingG), preparingG, g)
   151  }