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runtime: replace TestFutexsleep with TestTimediv

Browse Source 
TestFutexsleep was originally created in CL 7876043 as a
regression test for buggy division logic in futexsleep. Several months
later CL 11575044 moved this logic to timediv (called by futexsleep).

This test calls runtime.Futexsleep, which temporarily disables
asynchronous preemption. Unfortunately, TestFutexSleep calls this from
multiple goroutines, creating a race condition that may result in
asynchronous preemption remaining disabled for the remainder of the
process lifetime.

We could fix this by moving the async preemption disable to the main
test function, however this test has had a history of flakiness. As an
alternative, this CL replaces the test wholesale with a new test for
timediv, covering the overflow logic without the difficulty of dealing
with futex.

Fixes #50749.

Change-Id: If9e1dac63ef1535adb49f9a9ffcaff99b9135895
Reviewed-on: https://go-review.googlesource.com/c/go/+/380058
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Michael Pratt 2022-01-21 16:52:56 -05:00
parent f88c3b9f4d
commit 97e740e8b0
4 changed files with 78 additions and 106 deletions
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19
src/runtime/export_futex_test.go
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@ -1,19 +0,0 @@
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build dragonfly || freebsd || linux
package runtime
var Futexwakeup = futexwakeup
//go:nosplit
func Futexsleep(addr *uint32, val uint32, ns int64) {
// Temporarily disable preemption so that a preemption signal
// doesn't interrupt the system call.
poff := debug.asyncpreemptoff
debug.asyncpreemptoff = 1
futexsleep(addr, val, ns)
debug.asyncpreemptoff = poff
}

2
src/runtime/export_test.go
View File

@ -1328,3 +1328,5 @@ func Acquirem() {
func Releasem() {
releasem(getg().m)
}
var Timediv = timediv

87
src/runtime/futex_test.go
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@ -1,87 +0,0 @@
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Futex is only available on DragonFly BSD, FreeBSD and Linux.
// The race detector emits calls to split stack functions so it breaks
// the test.
//go:build (dragonfly || freebsd || linux) && !race
package runtime_test
import (
"runtime"
"sync"
"sync/atomic"
"testing"
"time"
)
type futexsleepTest struct {
mtx uint32
ns int64
msg string
ch chan *futexsleepTest
}
var futexsleepTests = []futexsleepTest{
beforeY2038: {mtx: 0, ns: 86400 * 1e9, msg: "before the year 2038"},
afterY2038: {mtx: 0, ns: (1<<31 + 100) * 1e9, msg: "after the year 2038"},
}
const (
beforeY2038 = iota
afterY2038
)
func TestFutexsleep(t *testing.T) {
if runtime.GOMAXPROCS(0) > 1 {
// futexsleep doesn't handle EINTR or other signals,
// so spurious wakeups may happen.
t.Skip("skipping; GOMAXPROCS>1")
}
start := time.Now()
var wg sync.WaitGroup
for i := range futexsleepTests {
tt := &futexsleepTests[i]
tt.mtx = 0
tt.ch = make(chan *futexsleepTest, 1)
wg.Add(1)
go func(tt *futexsleepTest) {
runtime.Entersyscall()
runtime.Futexsleep(&tt.mtx, 0, tt.ns)
runtime.Exitsyscall()
tt.ch <- tt
wg.Done()
}(tt)
}
loop:
for {
select {
case tt := <-futexsleepTests[beforeY2038].ch:
t.Errorf("futexsleep test %q finished early after %s", tt.msg, time.Since(start))
break loop
case tt := <-futexsleepTests[afterY2038].ch:
// Looks like FreeBSD 10 kernel has changed
// the semantics of timedwait on userspace
// mutex to make broken stuff look broken.
switch {
case runtime.GOOS == "freebsd" && runtime.GOARCH == "386":
t.Log("freebsd/386 may not work correctly after the year 2038, see golang.org/issue/7194")
default:
t.Errorf("futexsleep test %q finished early after %s", tt.msg, time.Since(start))
break loop
}
case <-time.After(time.Second):
break loop
}
}
for i := range futexsleepTests {
tt := &futexsleepTests[i]
atomic.StoreUint32(&tt.mtx, 1)
runtime.Futexwakeup(&tt.mtx, 1)
}
wg.Wait()
}

76
src/runtime/runtime_test.go
View File

@ -6,6 +6,7 @@ package runtime_test
import (
"flag"
"fmt"
"io"
. "runtime"
"runtime/debug"
@ -362,3 +363,78 @@ func TestVersion(t *testing.T) {
t.Fatalf("cr/nl in version: %q", vers)
}
}
func TestTimediv(t *testing.T) {
for _, tc := range []struct {
num int64
div int32
ret int32
rem int32
}{
{
num: 8,
div: 2,
ret: 4,
rem: 0,
},
{
num: 9,
div: 2,
ret: 4,
rem: 1,
},
{
// Used by runtime.check.
num: 12345*1000000000 + 54321,
div: 1000000000,
ret: 12345,
rem: 54321,
},
{
num: 1<<32 - 1,
div: 2,
ret: 1<<31 - 1, // no overflow.
rem: 1,
},
{
num: 1 << 32,
div: 2,
ret: 1<<31 - 1, // overflow.
rem: 0,
},
{
num: 1 << 40,
div: 2,
ret: 1<<31 - 1, // overflow.
rem: 0,
},
{
num: 1<<40 + 1,
div: 1 << 10,
ret: 1 << 30,
rem: 1,
},
} {
name := fmt.Sprintf("%d div %d", tc.num, tc.div)
t.Run(name, func(t *testing.T) {
// Double check that the inputs make sense using
// standard 64-bit division.
ret64 := tc.num / int64(tc.div)
rem64 := tc.num % int64(tc.div)
if ret64 != int64(int32(ret64)) {
// Simulate timediv overflow value.
ret64 = 1<<31 - 1
rem64 = 0
}
if ret64 != int64(tc.ret) {
t.Errorf("%d / %d got ret %d rem %d want ret %d rem %d", tc.num, tc.div, ret64, rem64, tc.ret, tc.rem)
}
var rem int32
ret := Timediv(tc.num, tc.div, &rem)
if ret != tc.ret || rem != tc.rem {
t.Errorf("timediv %d / %d got ret %d rem %d want ret %d rem %d", tc.num, tc.div, ret, rem, tc.ret, tc.rem)
}
})
}
}