Go: runtime: terminate locked OS thread if its goroutine exits

[rgooch]

I propose a new runtime.TaintOSThread function. This would mark the current thread that the calling goroutine is running on as being "tainted": not safe for use by other goroutines. When the goroutine exits or the goroutine is unpinned from the (previously pinned) OS thread, the thread is killed.

This is needed when goroutines are pinned to OS threads and the state of the thread is modified in way that is unsafe for other goroutines, such as unsharing and then modifying one of the Linux namespaces. Without this change, it is impossible to clean up OS threads: the pinning goroutines and OS threads need to stay until the programme exits, leading to unfixable goroutine and OS thread leaks.

The expected calling pattern is:

runtime.LockOSThread()
runtime.TaintOSThread()
syscall.Unshare(syscall.CLONE_NEWNS) // Example mutation of OS thread state.

[bradfitz]

Could you instead just LockOSThread + Unshare + then kill that thread yourself?

[rgooch]

How can I kill the thread safely? I don't see a function in the runtime package to do this. If I can safely kill the thread on which a goroutine is running, then that would work too, I just didn't think that would be as palatable as marking a thread as tainted.

There's actually no need to unshare (and that wouldn't help anyway, since the new namespace inherits from its ancestor).

[bradfitz]

syscall.Syscall(syscall.SYS_EXIT, 0, 0, 0)? Haven't tried it, though.

[bradfitz]

Works for me:

bradfitz@gdev:~$ cat exit.go

package main

import (
        "runtime"
        "syscall"
        "time"
)       

func main() {
        runtime.LockOSThread()
        println("Hi from ", syscall.Gettid())
        go func() {
                runtime.LockOSThread()
                println("Hi from ", syscall.Gettid())
                syscall.Syscall(syscall.SYS_EXIT, 0, 0, 0)
        }()     
        time.Sleep(500 * time.Millisecond)
        println("Bye from ", syscall.Gettid())
}    



md5-897b3321b3810b0cb2955882fcb1ef67



bradfitz@gdev:~$ go run exit.go
Hi from  6876
Hi from  6878
Bye from  6876

[bradfitz]

Or, better: (showing it actually went away)

package main

import (
        "fmt"
        "os"
        "runtime"
        "strconv"
        "syscall"
        "time"
)

func main() {
        runtime.LockOSThread()
        fmt.Println("Hi from ", syscall.Gettid())
        other := make(chan int, 1)
        go func() {
                runtime.LockOSThread()
                tid := syscall.Gettid()
                other <- tid
                fmt.Println("Hi from ", tid)
                fi, err := os.Stat("/proc/" + strconv.Itoa(tid))
                fmt.Printf("proc %d: %v, %v\n", tid, fi, err)
                syscall.Syscall(syscall.SYS_EXIT, 0, 0, 0)
        }()
        time.Sleep(500 * time.Millisecond)
        tid := <-other
        fi, err := os.Stat("/proc/" + strconv.Itoa(tid))
        fmt.Printf("proc %d: %v, %v\n", tid, fi, err)
        fmt.Println("Bye from ", syscall.Gettid())
}

Hi from  7277
Hi from  7279
proc 7279: &{7279 0 2147484013 {63630648429 114580211 0x550860} 0xc20804e000}, <nil>
proc 7279: <nil>, stat /proc/7279: no such file or directory
Bye from  7277

[randall77]

I worry that the runtime would be upset at an OS thread permanently vanishing. The runtime may want notification to ensure that anything the m is holding is freed first.
Maybe the existing enterSysCall already does everything that might matter?
At the very least, the allm list will grow without bound.

[rgooch]

Yes, that was one of my worries too. Can we get some kind of guidance on whether it's considered safe to kill a thread? What happens to the goroutine that's running on it?

[randall77]

@aclements

[ianlancetaylor]

I do think we need to let the runtime know that the thread can not be used. The simple approach is

runtime.LockOSThread()
select{}

That will ensure that nothing else uses the thread, but it won't actually kill the thread. It would be an acceptable approach if you only need to do this a few times, but of course would be unsatisfactory if you need to do it continually.

Another approach would be to go through C. Have C code create a new thread, which could then call into Go. That gives you a thread that the Go runtime won't use itself, so you can simply call pthread_exit when done. Of course this is not ideal if you otherwise have a pure Go program.

[rgooch]

All my Go code so far is pure Go. I was hoping to keep it that way.

It seems to me that we need some way to cleanly kill a thread. The hack shown above is, well, a hack :-)

[ianlancetaylor]

Is runtime.ThreadExit sufficient for your purposes? I'm thinking of a function that would simply cause the thread to exit and not return. The goroutine could continue running on a different thread.

I'll note that I appreciate that you need this, but it seems awfully special purpose.

[rgooch]

Yes, runtime.ThreadExit would work fine. I'm not special, I'm just ahead of the curve :-)

[bradfitz]

@ianlancetaylor, or instead of introducing new API: just consider all threads "tainted" if they ever used LockOSThread and when the goroutine exits (implicitly or via the existing runtime.Goexit()), then kill the thread.

[rgooch]

That works for me too.

[rasky]

Or maybe consider tainted all locked threads whose goroutine exits. If a goroutine calls Unlock before exiting, the thread is probably safe for reuse.

[rgooch]

Yes, that works too. Which of these proposals is the most palatable?

[aclements]

I rather like @rasky's suggestion. If a goroutine exits without unlocking its thread, then you could think of that thread as still being locked, but with nothing it can ever do again, so you might as well kill it.

My only concern would be existing code that exits without unlocking implicitly unlocks the thread right now, so this change could cause such code to cycle through new OS threads, which could be a performance problem. But this all seems sufficiently obscure (and the fix for this performance problem is trivial) that I'm not too concerned.

[rgooch]

Indeed. Can we can get an approval from the core team for the proposal that @rasky made?

[bradfitz]

@rgooch, we do proposal reviews roughly weekly, on Mondays. Sometimes we don't get through all open proposals and things wait another week.

[rgooch]

Did this get into the review this week?

[bradfitz]

Nope. We spent this week on NeedsDecision bugs (#20192).

When this gets reviewed, updates will be posted here.

[aclements]

@rsc, @dr2chase, @ianlancetaylor and myself are all good with @rasky's suggestion. I haven't figured out how difficult it is to implement yet (it's certainly too late for Go 1.9).

[gopherbot]

CL https://golang.org/cl/46038 mentions this issue.

[gopherbot]

CL https://golang.org/cl/46037 mentions this issue.

[rgooch]

Well, this is nice to see. Thank you. So, it looks like this will be available in go1.10 but not go1.9?

How can one add a compile assert so that a minimum go version is required to compile? Once this is available, I can relax certain restrictions in my code, but I need to be sure I have this fix otherwise it will be unsafe.

[ianlancetaylor]

Use build tags, as in

// +build go1.10

to only compile a file for Go 1.10.

See "Build Constraints" in https://golang.org/pkg/go/build/ .

[benschumacher]

Maybe an aside to this behavior change, but is there a definition of what a "clean" thread would be, w.r.t. to the current state. The premise here is that the developer will be responsible for returning a locked thread to some reusable state, but it's clear what this means in a practical sense.

Perhaps this is left to the end user, but it'd be good to have guidelines in documentation re: the basic assumptions in this implicit behavior.

[rgooch]

It's not possible to guarantee that a thread is returned to a previous state. For example, when unsharing a namespace, you may not be able to rejoin the original namespace. Thus, I think the safe approach is "once tainted, forever tainted. Kill it. Kill it with fire. Nuke it from orbit"

[aclements]

Maybe an aside to this behavior change, but is there a definition of what a "clean" thread would be, w.r.t. to the current state.

Really any change to thread state makes it unclean. That could mean namespace, priority, affinity, values in TLS, and probably at least a dozen other things that vary from OS to OS. If it's in any way distinguishable from any other thread in your process, it's probably not safe to return it to the thread pool. I'm not sure how to give useful guidelines on this, though I'm open to suggestions.

[rgooch]

Why even worry about this? The simple and safe thing to do is kill the thread. Creating a new thread is cheap enough. I doubt that threads are being tainted at a high rate, so the cost of creating a new thread is insignificant in this context.

[benschumacher]

Why even worry about this? The simple and safe thing to do is kill the thread. Creating a new thread is cheap enough. I doubt that threads are being tainted at a high rate, so the cost of creating a new thread is insignificant in this context.

Sure. To be clear, I'm not suggesting that user's are encouraged to try to reset thread state. I agree that tracking / observing all potential those changes is highly complex and dependent on operating environment. The alternative of just leading thread be reaped / recreate is almost certainly more efficient from runtime and development perspective.

I'm not sure how to give useful guidelines on this, though I'm open to suggestions.

I suppose the documentation could be a note on LockOSThread:

A goroutine's thread context can be isolated with LockOSThread to allow modifications to that thread's system-level context, such as namespace, uid, gid, etc. Leaving the OS Thread locked at the completion of the goroutine indicates that the thread is polluted and should not be reused by other goroutines.

(Perhaps you had already come to something along these lines, but I do think providing some context in docs of the use case for this is valuable.) Cheers!

[aclements]

@benschumacher, I took a stab at your suggestion here: https://go-review.googlesource.com/c/52871

[markdascher]

May also be worth auditing references to UnlockOSThread to make sure they're being used appropriately. This one from ensureSigM seems debatable. (The goroutine never exits, as far as I can tell. But if it did, you wouldn't want the thread to be reused.)

[gopherbot]

Change https://golang.org/cl/68750 mentions this issue: runtime: rename sched.mcount to sched.mnext

[gopherbot]

Change https://golang.org/cl/85662 mentions this issue: runtime: remove special handling of g0 stack

[rgooch]

@aclements: I'm reading the release notes for go1.10rc1 about locked OS threads. This part in particular has me concerned:

Because one common use of LockOSThread and UnlockOSThread is to allow Go code to reliably modify thread-local state (for example, Linux or Plan 9 name spaces), the runtime now treats locked threads as unsuitable for reuse or for creating new threads.

I have code which locks the thread, unshares the mount namespace, mounts a file-system (which is now visible only in that thread) and calls os/exec to run commands which _need access to the mounted file-system_. The code relies on the forked process being in the same mount namespace as the thread.

Will this code break with the new behaviour? It's unclear from the release notes.

[aclements]

Hi @rgooch. This change is specifically to make things like that better. Prior to this change, random goroutines could wind up running in your new mount namespace because the runtime spawned new internal threads off your locked thread. Now that will no longer happen. But this doesn't affect the behavior of os/exec.

Maybe I misunderstand your concern? If the release notes aren't clear, I'd like to understand how to improve them. Is your specific interpretation of the release notes that this would affect the fork performed by os/exec because that, in some sense, creates a new thread? What we really mean is that the runtime will never clone another thread off of a locked thread for the purposes of scheduling goroutines.

[rgooch]

Hi, @aclements. Correct, the release notes are not clear to me. From your explanation above, it seems that when using os/exec, the thread state of the calling goroutine is inherited, regardless of whether the goroutine called runime.LockOSThread (that's the behaviour I rely on). It would help if this were made clear.

[aclements]

Yes, with os/exec, the thread state of the caller is inherited by the new process. Though it's still important to call runtime.LockOSThread because otherwise you have no control over which thread's state will be inherited.

[rgooch]

OK, great. If you can tweak the documentation and release notes to make that clear, that will be helpful, thanks. The behaviour you've described is the one I rely on, and it's good to know I can keep relying on :-)