mirror of
https://github.com/golang/go
synced 2024-09-28 18:14:29 -06:00
50d6c81d4a
(Thanks to ken and rsc for pointing this out)
rsc:
ken pointed out that there's a race in the new
one-lock-per-channel code. the issue is that
if one goroutine has gone to sleep doing
select {
case <-c1:
case <-c2:
}
and then two more goroutines try to send
on c1 and c2 simultaneously, the way that
the code makes sure only one wins is the
selgen field manipulation in dequeue:
// if sgp is stale, ignore it
if(sgp->selgen != sgp->g->selgen) {
//prints("INVALID PSEUDOG POINTER\n");
freesg(c, sgp);
goto loop;
}
// invalidate any others
sgp->g->selgen++;
but because the global lock is gone both
goroutines will be fiddling with sgp->g->selgen
at the same time.
This results in a 7% slowdown in the single threaded case for a
ping-pong microbenchmark.
Since the cas predominantly succeeds, adding a simple check first
didn't make any difference.
R=rsc
CC=golang-dev
https://golang.org/cl/180068
84 lines
1.9 KiB
Go
84 lines
1.9 KiB
Go
// $G $D/$F.go && $L $F.$A && ./$A.out
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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// This test is designed to flush out the case where two cases of a select can
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// both end up running. See http://codereview.appspot.com/180068.
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package main
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import (
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"flag"
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"runtime"
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)
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var iterations *int = flag.Int("n", 100000, "number of iterations")
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// sender sends a counter to one of four different channels. If two
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// cases both end up running in the same iteration, the same value will be sent
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// to two different channels.
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func sender(n int, c1, c2, c3, c4 chan<- int) {
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defer close(c1)
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defer close(c2)
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for i := 0; i < n; i++ {
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select {
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case c1 <- i:
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case c2 <- i:
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case c3 <- i:
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case c4 <- i:
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}
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}
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}
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// mux receives the values from sender and forwards them onto another channel.
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// It would be simplier to just have sender's four cases all be the same
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// channel, but this doesn't actually trigger the bug.
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func mux(out chan<- int, in <-chan int) {
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for {
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v := <-in
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if closed(in) {
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close(out)
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break
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}
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out <- v
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}
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}
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// recver gets a steam of values from the four mux's and checks for duplicates.
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func recver(in <-chan int) {
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seen := make(map[int]bool)
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for {
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v := <-in
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if closed(in) {
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break
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}
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if _, ok := seen[v]; ok {
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panic("got duplicate value: ", v)
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}
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seen[v] = true
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}
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}
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func main() {
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runtime.GOMAXPROCS(2)
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c1 := make(chan int)
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c2 := make(chan int)
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c3 := make(chan int)
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c4 := make(chan int)
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cmux := make(chan int)
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go sender(*iterations, c1, c2, c3, c4)
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go mux(cmux, c1)
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go mux(cmux, c2)
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go mux(cmux, c3)
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go mux(cmux, c4)
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// We keep the recver because it might catch more bugs in the future.
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// However, the result of the bug linked to at the top is that we'll
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// end up panicing with: "throw: bad g->status in ready".
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recver(cmux)
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print("PASS\n")
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}
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