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remyoudompheng opened this issue Nov 21, 2012 · 12 comments
Closed

sync/atomic: arm CompareAndSwap backtraces fail #4422

remyoudompheng opened this issue Nov 21, 2012 · 12 comments
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Go1.1

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@remyoudompheng
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What steps will reproduce the problem?
1. go run the following program

package main

import (
      "sync"
)

func main() {
      var lock *sync.Mutex
      lock.Lock()
}

What is the expected output? What do you see instead?

Expected: something looking roughly like the x86 case

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x4d900d]

goroutine 1 [running]:
sync/atomic.CompareAndSwapUint32(0x0, 0x100000000, 0xc200079870, 0x4, 0x4d6d95, ...)
    /opt/remy/go/src/pkg/sync/atomic/asm_amd64.s:14 +0xd
sync.(*Mutex).Lock(0x0, 0x76eda8)
    /opt/remy/go/src/pkg/sync/mutex.go:43 +0x35
sync.(*RWMutex).Lock(0x0, 0x0)
    /opt/remy/go/src/pkg/sync/rwmutex.go:75 +0x25
net/http.(*ServeMux).Handle(0x0, 0x6247a0, 0x4, 0xc200079870, 0x400c50, ...)
    /opt/remy/go/src/pkg/net/http/server.go:998 +0x30
net/http.(*ServeMux).HandleFunc(0x0, 0x6247a0, 0x4, 0x400c50, 0x400c84, ...)
    /opt/remy/go/src/pkg/net/http/server.go:1036 +0x58
main.Register(0x0, 0x400f5a)
    /home/remy/travail/go/src/teststack/irc.go:8 +0x43
main.init·1()
    /home/remy/travail/go/src/teststack/webtoys.go:15 +0x24
main.init()
    /home/remy/travail/go/src/teststack/webtoys.go:33 +0xaa

goroutine 2 [syscall]:
created by runtime.main
    /opt/remy/go/src/pkg/runtime/proc.c:225

Got: empty stack trace

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0xffff0fc4]

goroutine 1 [running]:

goroutine 2 [syscall]:
created by runtime.main
    /storage/remy/go/src/pkg/runtime/proc.c:225
exit status 2

Also gdb shows corrupted output:

(gdb) run
Starting program: /home/remy/go/src/teststack/lock 
[New LWP 6399]

Program received signal SIGSEGV, Segmentation fault.
[Switching to LWP 6399]
0xffff0fc4 in ?? ()
(gdb) bt
#0  0xffff0fc4 in ?? ()
#1  0x00025150 in sync/atomic.CompareAndSwapUint32 (addr=void, old=void, new=void,
ret=void)
    at /storage/remy/go/src/pkg/sync/atomic/asm_linux_arm.s:36
#2  0x00025150 in sync/atomic.CompareAndSwapUint32 (addr=void, old=void, new=void,
ret=void)
    at /storage/remy/go/src/pkg/sync/atomic/asm_linux_arm.s:36
#3  0x00025150 in sync/atomic.CompareAndSwapUint32 (addr=void, old=void, new=void,
ret=void)
    at /storage/remy/go/src/pkg/sync/atomic/asm_linux_arm.s:36
#4  0x00025150 in sync/atomic.CompareAndSwapUint32 (addr=void, old=void, new=void,
ret=void)
    at /storage/remy/go/src/pkg/sync/atomic/asm_linux_arm.s:36
#5  0x00025150 in sync/atomic.CompareAndSwapUint32 (addr=void, old=void, new=void,
ret=void)
    at /storage/remy/go/src/pkg/sync/atomic/asm_linux_arm.s:36


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@remyoudompheng
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Comment 1:

The issue initially arose when calling HandleFunc on a nil *http.ServeMux.

@minux
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minux commented Nov 25, 2012

Comment 2:

the sigsegv happened in kernel provided cas primitive and it doesn't use our standard
stack frame so it confused the back tracer.
perhaps we can force a read in sync/atomic CompareAndSwapUint32 so the the signal
is received earlier.
diff -r bbc0024af4a4 src/pkg/sync/atomic/asm_linux_arm.s
--- a/src/pkg/sync/atomic/asm_linux_arm.s       Sun Nov 18 15:31:26 2012 +1100
+++ b/src/pkg/sync/atomic/asm_linux_arm.s       Sun Nov 25 00:20:48 2012 +0000
@@ -30,6 +30,7 @@
 // Implement using kernel cas for portability.
 TEXT ·CompareAndSwapUint32(SB),7,$0
        MOVW    addr+0(FP), R2
+       MOVW    (R2), R1 // force a read here
        MOVW    old+4(FP), R0
 casagain:
        MOVW    new+8(FP), R1

Labels changed: added priority-later, go1.1, arch-arm, removed priority-triage, go1.1maybe.

Status changed to Accepted.

@remyoudompheng
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Comment 3:

Yes, that's what I did locally and it's much better.

@minux
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minux commented Nov 25, 2012

Comment 4:

as an alternative, we can fix the backtracer to recognize this case.
which solution do you think is better?

@remyoudompheng
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Comment 5:

You would need to fix both Go's backtracer and GDB's backtracer then. Do you know how to
do that?
If it gets too complicated, I would say a dummy instruction in the ASM would be useful.
I don't know what the performance impact would be.

@davecheney
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Comment 6:

I think we should to Remy's solution, a single MOVW won't hurt performance, the call
stack for Arm's CAS is quite long.

@rsc
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rsc commented Dec 10, 2012

Comment 7:

Rémy, want to send this 1-line fix as a CL? LGTM.

@rsc
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rsc commented Dec 10, 2012

Comment 9:

Labels changed: added size-s.

@davecheney
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Comment 10:

https://golang.org/cl/6939056

Owner changed to @davecheney.

Status changed to Started.

@davecheney
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Comment 11:

I've not been able to solve the problem for the CAS64 case, if others want to pick this
up, please consider https://golang.org/cl/6939056/ as a starting point.

Owner changed to ---.

Status changed to Accepted.

@rsc
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rsc commented Feb 14, 2013

Comment 12:

This was fixed by 
changeset:   15628:8f547c432c68
user:        Shenghou Ma <minux.ma@gmail.com>
date:        Wed Feb 06 01:18:37 2013 +0800
summary:     runtime: save LR to stack when panicking to handle leaf function traceback
right?

@minux
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minux commented Feb 14, 2013

Comment 13:

right.

Status changed to Fixed.

@rsc rsc added this to the Go1.1 milestone Apr 14, 2015
@rsc rsc removed the go1.1 label Apr 14, 2015
@golang golang locked and limited conversation to collaborators Jun 24, 2016
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@davecheney @rsc @minux @remyoudompheng @gopherbot