rwmutex.go

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  import (
     8  	"runtime/internal/atomic"
     9  )
    10  
    11  // This is a copy of sync/rwmutex.go rewritten to work in the runtime.
    12  
    13  // A rwmutex is a reader/writer mutual exclusion lock.
    14  // The lock can be held by an arbitrary number of readers or a single writer.
    15  // This is a variant of sync.RWMutex, for the runtime package.
    16  // Like mutex, rwmutex blocks the calling M.
    17  // It does not interact with the goroutine scheduler.
    18  type rwmutex struct {
    19  	rLock      mutex    // protects readers, readerPass, writer
    20  	readers    muintptr // list of pending readers
    21  	readerPass uint32   // number of pending readers to skip readers list
    22  
    23  	wLock  mutex    // serializes writers
    24  	writer muintptr // pending writer waiting for completing readers
    25  
    26  	readerCount uint32 // number of pending readers
    27  	readerWait  uint32 // number of departing readers
    28  }
    29  
    30  const rwmutexMaxReaders = 1 << 30
    31  
    32  // rlock locks rw for reading.
    33  func (rw *rwmutex) rlock() {
    34  	// The reader must not be allowed to lose its P or else other
    35  	// things blocking on the lock may consume all of the Ps and
    36  	// deadlock (issue #20903). Alternatively, we could drop the P
    37  	// while sleeping.
    38  	acquirem()
    39  	if int32(atomic.Xadd(&rw.readerCount, 1)) < 0 {
    40  		// A writer is pending. Park on the reader queue.
    41  		systemstack(func() {
    42  			lockWithRank(&rw.rLock, lockRankRwmutexR)
    43  			if rw.readerPass > 0 {
    44  				// Writer finished.
    45  				rw.readerPass -= 1
    46  				unlock(&rw.rLock)
    47  			} else {
    48  				// Queue this reader to be woken by
    49  				// the writer.
    50  				m := getg().m
    51  				m.schedlink = rw.readers
    52  				rw.readers.set(m)
    53  				unlock(&rw.rLock)
    54  				notesleep(&m.park)
    55  				noteclear(&m.park)
    56  			}
    57  		})
    58  	}
    59  }
    60  
    61  // runlock undoes a single rlock call on rw.
    62  func (rw *rwmutex) runlock() {
    63  	if r := int32(atomic.Xadd(&rw.readerCount, -1)); r < 0 {
    64  		if r+1 == 0 || r+1 == -rwmutexMaxReaders {
    65  			throw("runlock of unlocked rwmutex")
    66  		}
    67  		// A writer is pending.
    68  		if atomic.Xadd(&rw.readerWait, -1) == 0 {
    69  			// The last reader unblocks the writer.
    70  			lockWithRank(&rw.rLock, lockRankRwmutexR)
    71  			w := rw.writer.ptr()
    72  			if w != nil {
    73  				notewakeup(&w.park)
    74  			}
    75  			unlock(&rw.rLock)
    76  		}
    77  	}
    78  	releasem(getg().m)
    79  }
    80  
    81  // lock locks rw for writing.
    82  func (rw *rwmutex) lock() {
    83  	// Resolve competition with other writers and stick to our P.
    84  	lockWithRank(&rw.wLock, lockRankRwmutexW)
    85  	m := getg().m
    86  	// Announce that there is a pending writer.
    87  	r := int32(atomic.Xadd(&rw.readerCount, -rwmutexMaxReaders)) + rwmutexMaxReaders
    88  	// Wait for any active readers to complete.
    89  	lockWithRank(&rw.rLock, lockRankRwmutexR)
    90  	if r != 0 && atomic.Xadd(&rw.readerWait, r) != 0 {
    91  		// Wait for reader to wake us up.
    92  		systemstack(func() {
    93  			rw.writer.set(m)
    94  			unlock(&rw.rLock)
    95  			notesleep(&m.park)
    96  			noteclear(&m.park)
    97  		})
    98  	} else {
    99  		unlock(&rw.rLock)
   100  	}
   101  }
   102  
   103  // unlock unlocks rw for writing.
   104  func (rw *rwmutex) unlock() {
   105  	// Announce to readers that there is no active writer.
   106  	r := int32(atomic.Xadd(&rw.readerCount, rwmutexMaxReaders))
   107  	if r >= rwmutexMaxReaders {
   108  		throw("unlock of unlocked rwmutex")
   109  	}
   110  	// Unblock blocked readers.
   111  	lockWithRank(&rw.rLock, lockRankRwmutexR)
   112  	for rw.readers.ptr() != nil {
   113  		reader := rw.readers.ptr()
   114  		rw.readers = reader.schedlink
   115  		reader.schedlink.set(nil)
   116  		notewakeup(&reader.park)
   117  		r -= 1
   118  	}
   119  	// If r > 0, there are pending readers that aren't on the
   120  	// queue. Tell them to skip waiting.
   121  	rw.readerPass += uint32(r)
   122  	unlock(&rw.rLock)
   123  	// Allow other writers to proceed.
   124  	unlock(&rw.wLock)
   125  }
   126
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