Future Technology

Debugging a FUSE impasse within the Linux kernel | by Netflix Know-how Weblog | Could, 2023

11 min read

Tycho Andersen

The Compute workforce at Netflix is charged with managing all AWS and containerized workloads at Netflix, together with autoscaling, deployment of containers, subject remediation, and so forth. As a part of this workforce, I work on fixing unusual issues that customers report.

This explicit subject concerned a customized inside FUSE filesystem: ndrive. It had been festering for a while, however wanted somebody to sit down down and take a look at it in anger. This weblog publish describes how I poked at /procto get a way of what was occurring, earlier than posting the problem to the kernel mailing checklist and getting schooled on how the kernel’s wait code truly works!

We had a caught docker API name:

goroutine 146 [select, 8817 minutes]:
internet/http.(*persistConn).roundTrip(0xc000658fc0, 0xc0003fc080, 0x0, 0x0, 0x0)
/usr/native/go/src/internet/http/transport.go:2610 +0x765
internet/http.(*Transport).roundTrip(0xc000420140, 0xc000966200, 0x30, 0x1366f20, 0x162)
/usr/native/go/src/internet/http/transport.go:592 +0xacb
internet/http.(*Transport).RoundTrip(0xc000420140, 0xc000966200, 0xc000420140, 0x0, 0x0)
/usr/native/go/src/internet/http/roundtrip.go:17 +0x35
internet/http.ship(0xc000966200, 0x161eba0, 0xc000420140, 0x0, 0x0, 0x0, 0xc00000e050, 0x3, 0x1, 0x0)
/usr/native/go/src/internet/http/shopper.go:251 +0x454
internet/http.(*Consumer).ship(0xc000438480, 0xc000966200, 0x0, 0x0, 0x0, 0xc00000e050, 0x0, 0x1, 0x10000168e)
/usr/native/go/src/internet/http/shopper.go:175 +0xff
internet/http.(*Consumer).do(0xc000438480, 0xc000966200, 0x0, 0x0, 0x0)
/usr/native/go/src/internet/http/shopper.go:717 +0x45f
/usr/native/go/src/internet/http/shopper.go:585, 0xc000044090, 0xc000438480, 0xc000966100, 0x0, 0x0, 0x0)
/go/pkg/mod/[email protected]/context/ctxhttp/ctxhttp.go:27 +0x10f*Consumer).doRequest(0xc0001a8200, 0x163bd48, 0xc000044090, 0xc000966100, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/go/pkg/mod/[email protected]/shopper/request.go:132 +0xbe*Consumer).sendRequest(0xc0001a8200, 0x163bd48, 0xc000044090, 0x13d8643, 0x3, 0xc00079a720, 0x51, 0x0, 0x0, 0x0, ...)
/go/pkg/mod/[email protected]/shopper/request.go:122 +0x156*Consumer).get(...)
/go/pkg/mod/[email protected]/shopper/request.go:37*Consumer).ContainerInspect(0xc0001a8200, 0x163bd48, 0xc000044090, 0xc0006a01c0, 0x40, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/go/pkg/mod/[email protected]/shopper/container_inspect.go:18 +0x128*DockerRuntime).Kill(0xc000215180, 0x163bdb8, 0xc000938600, 0x1, 0x0, 0x0)
/var/lib/buildkite-agent/builds/ip-192-168-1-90-1/netflix/titus-executor/executor/runtime/docker/docker.go:2835 +0x310*Runner).doShutdown(0xc000432dc0, 0x163bd10, 0xc000938390, 0x1, 0xc000b821e0, 0x1d, 0xc0005e4710)
/var/lib/buildkite-agent/builds/ip-192-168-1-90-1/netflix/titus-executor/executor/runner/runner.go:326 +0x4f4*Runner).startRunner(0xc000432dc0, 0x163bdb8, 0xc00071e0c0, 0xc0a502e28c08b488, 0x24572b8, 0x1df5980)
/var/lib/buildkite-agent/builds/ip-192-168-1-90-1/netflix/titus-executor/executor/runner/runner.go:122 +0x391
created by
/var/lib/buildkite-agent/builds/ip-192-168-1-90-1/netflix/titus-executor/executor/runner/runner.go:81 +0x411

Right here, our administration engine has made an HTTP name to the Docker API’s unix socket asking it to kill a container. Our containers are configured to be killed through SIGKILL. However that is unusual. kill(SIGKILL) needs to be comparatively deadly, so what’s the container doing?

$ docker exec -it 6643cd073492 bash
OCI runtime exec failed: exec failed: container_linux.go:380: beginning container course of precipitated: process_linux.go:130: executing setns course of precipitated: exit standing 1: unknown

Hmm. Looks like it’s alive, however setns(2) fails. Why would that be? If we take a look at the method tree through ps awwfux, we see:

_ containerd-shim -namespace moby -workdir /var/lib/containerd/io.containerd.runtime.v1.linux/moby/6643cd073492ba9166100ed30dbe389ff1caef0dc3d35
| _ [docker-init]
| _ [ndrive] <defunct>

Okay, so the container’s init course of remains to be alive, nevertheless it has one zombie youngster. What might the container’s init course of presumably be doing?

# cat /proc/1528591/stack
[<0>] do_wait+0x156/0x2f0
[<0>] kernel_wait4+0x8d/0x140
[<0>] zap_pid_ns_processes+0x104/0x180
[<0>] do_exit+0xa41/0xb80
[<0>] do_group_exit+0x3a/0xa0
[<0>] __x64_sys_exit_group+0x14/0x20
[<0>] do_syscall_64+0x37/0xb0
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xae

It’s within the means of exiting, nevertheless it appears caught. The one youngster is the ndrive course of in Z (i.e. “zombie”) state, although. Zombies are processes which have efficiently exited, and are ready to be reaped by a corresponding wait() syscall from their dad and mom. So how might the kernel be caught ready on a zombie?

# ls /proc/1544450/activity
1544450 1544574

Ah ha, there are two threads within the thread group. One in all them is a zombie, perhaps the opposite one isn’t:

# cat /proc/1544574/stack
[<0>] request_wait_answer+0x12f/0x210
[<0>] fuse_simple_request+0x109/0x2c0
[<0>] fuse_flush+0x16f/0x1b0
[<0>] filp_close+0x27/0x70
[<0>] put_files_struct+0x6b/0xc0
[<0>] do_exit+0x360/0xb80
[<0>] do_group_exit+0x3a/0xa0
[<0>] get_signal+0x140/0x870
[<0>] arch_do_signal_or_restart+0xae/0x7c0
[<0>] exit_to_user_mode_prepare+0x10f/0x1c0
[<0>] syscall_exit_to_user_mode+0x26/0x40
[<0>] do_syscall_64+0x46/0xb0
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xae

Certainly it isn’t a zombie. It’s making an attempt to change into one as arduous as it will probably, nevertheless it’s blocking inside FUSE for some purpose. To search out out why, let’s take a look at some kernel code. If we take a look at zap_pid_ns_processes(), it does:

* Reap the EXIT_ZOMBIE kids we had earlier than we ignored SIGCHLD.
* kernel_wait4() will even block till our kids traced from the
* mother or father namespace are indifferent and change into EXIT_DEAD.
rc = kernel_wait4(-1, NULL, __WALL, NULL);
whereas (rc != -ECHILD);

which is the place we’re caught, however earlier than that, it has completed:

/* Do not permit any extra processes into the pid namespace */

which is why docker can’t setns() — the namespace is a zombie. Okay, so we will’t setns(2), however why are we caught in kernel_wait4()? To grasp why, let’s take a look at what the opposite thread was doing in FUSE’s request_wait_answer():

* Both request is already in userspace, or it was pressured.
* Wait it out.
wait_event(req->waitq, test_bit(FR_FINISHED, &req->flags));

Okay, so we’re ready for an occasion (on this case, that userspace has replied to the FUSE flush request). However zap_pid_ns_processes()despatched a SIGKILL! SIGKILL needs to be very deadly to a course of. If we take a look at the method, we will certainly see that there’s a pending SIGKILL:

# grep Pnd /proc/1544574/standing
SigPnd: 0000000000000000
ShdPnd: 0000000000000100

Viewing course of standing this fashion, you may see 0x100 (i.e. the ninth bit is ready) below ShdPnd, which is the sign quantity similar to SIGKILL. Pending alerts are alerts which have been generated by the kernel, however haven’t but been delivered to userspace. Alerts are solely delivered at sure instances, for instance when coming into or leaving a syscall, or when ready on occasions. If the kernel is presently doing one thing on behalf of the duty, the sign could also be pending. Alerts will also be blocked by a activity, in order that they’re by no means delivered. Blocked alerts will present up of their respective pending units as nicely. Nevertheless, man 7 sign says: “The alerts SIGKILL and SIGSTOP can’t be caught, blocked, or ignored.” However right here the kernel is telling us that we have now a pending SIGKILL, aka that it’s being ignored even whereas the duty is ready!

Effectively that’s bizarre. The wait code (i.e. embrace/linux/wait.h) is used all over the place within the kernel: semaphores, wait queues, completions, and so forth. Certainly it is aware of to search for SIGKILLs. So what does wait_event() truly do? Digging by the macro expansions and wrappers, the meat of it’s:

#outline ___wait_event(wq_head, situation, state, unique, ret, cmd)           
__label__ __out;
struct wait_queue_entry __wq_entry;
lengthy __ret = ret; /* express shadow */

init_wait_entry(&__wq_entry, unique ? WQ_FLAG_EXCLUSIVE : 0);
for (;;)
lengthy __int = prepare_to_wait_event(&wq_head, &__wq_entry, state);

if (situation)

if (___wait_is_interruptible(state) && __int)
__ret = __int;
goto __out;


finish_wait(&wq_head, &__wq_entry);
__out: __ret;

So it loops perpetually, doing prepare_to_wait_event(), checking the situation, then checking to see if we have to interrupt. Then it does cmd, which on this case is schedule(), i.e. “do one thing else for some time”. prepare_to_wait_event() seems like:

lengthy prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)

unsigned lengthy flags;
lengthy ret = 0;

spin_lock_irqsave(&wq_head->lock, flags);
if (signal_pending_state(state, present))
* Unique waiter should not fail if it was chosen by wakeup,
* it ought to "eat" the situation we had been ready for.
* The caller will recheck the situation and return success if
* we had been already woken up, we can't miss the occasion as a result of
* wakeup locks/unlocks the identical wq_head->lock.
* However we have to make sure that set-condition + wakeup after that
* cannot see us, it ought to get up one other unique waiter if
* we fail.
if (list_empty(&wq_entry->entry))
if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
__add_wait_queue_entry_tail(wq_head, wq_entry);
__add_wait_queue(wq_head, wq_entry);


spin_unlock_irqrestore(&wq_head->lock, flags);

return ret;


It seems like the one method we will get away of this with a non-zero exit code is that if signal_pending_state() is true. Since our name web site was simply wait_event(), we all know that state right here is TASK_UNINTERRUPTIBLE; the definition of signal_pending_state() seems like:

static inline int signal_pending_state(unsigned int state, struct task_struct *p)

Our activity just isn’t interruptible, so the primary if fails. Our activity ought to have a sign pending, although, proper?

static inline int signal_pending(struct task_struct *p)

* TIF_NOTIFY_SIGNAL is not actually a sign, nevertheless it requires the identical
* conduct by way of making certain that we get away of wait loops
* in order that notify sign callbacks may be processed.
if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
return 1;
return task_sigpending(p);

Because the remark notes, TIF_NOTIFY_SIGNAL isn’t related right here, regardless of its identify, however let’s take a look at task_sigpending():

static inline int task_sigpending(struct task_struct *p)

return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));

Hmm. Looks like we must always have that flag set, proper? To determine that out, let’s take a look at how sign supply works. Once we’re shutting down the pid namespace in zap_pid_ns_processes(), it does:

group_send_sig_info(SIGKILL, SEND_SIG_PRIV, activity, PIDTYPE_MAX);

which ultimately will get to __send_signal_locked(), which has:

pending = (kind != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
sigaddset(&pending->sign, sig);
complete_signal(sig, t, kind);

Utilizing PIDTYPE_MAX right here as the sort is a bit of bizarre, nevertheless it roughly signifies “that is very privileged kernel stuff sending this sign, you must undoubtedly ship it”. There’s a little bit of unintended consequence right here, although, in that __send_signal_locked() finally ends up sending the SIGKILL to the shared set, as an alternative of the person activity’s set. If we take a look at the __fatal_signal_pending() code, we see:

static inline int __fatal_signal_pending(struct task_struct *p)

return unlikely(sigismember(&p->pending.sign, SIGKILL));

However it seems this can be a little bit of a crimson herring (although it took a while for me to grasp that).

To grasp what’s actually occurring right here, we have to take a look at complete_signal(), because it unconditionally provides a SIGKILL to the duty’s pending set:

sigaddset(&t->pending.sign, SIGKILL);

however why doesn’t it work? On the prime of the perform we have now:

* Now discover a thread we will get up to take the sign off the queue.
* If the primary thread needs the sign, it will get first crack.
* Most likely the least stunning to the common bear.
if (wants_signal(sig, p))
t = p;
else if ((kind == PIDTYPE_PID) || thread_group_empty(p))
* There is only one thread and it doesn't should be woken.
* It should dequeue unblocked alerts earlier than it runs once more.

however as Eric Biederman described, mainly each thread can deal with a SIGKILL at any time. Right here’s wants_signal():

static inline bool wants_signal(int sig, struct task_struct *p)

if (sigismember(&p->blocked, sig))
return false;

if (p->flags & PF_EXITING)
return false;

if (sig == SIGKILL)
return true;

if (task_is_stopped_or_traced(p))
return false;

return task_curr(p)

So… if a thread is already exiting (i.e. it has PF_EXITING), it doesn’t desire a sign. Take into account the next sequence of occasions:

1. a activity opens a FUSE file, and doesn’t shut it, then exits. Throughout that exit, the kernel dutifully calls do_exit(), which does the next:

exit_signals(tsk); /* units PF_EXITING */

2. do_exit() continues on to exit_files(tsk);, which flushes all recordsdata which are nonetheless open, ensuing within the stack hint above.

3. the pid namespace exits, and enters zap_pid_ns_processes(), sends a SIGKILL to everybody (that it expects to be deadly), after which waits for everybody to exit.

4. this kills the FUSE daemon within the pid ns so it will probably by no means reply.

5. complete_signal() for the FUSE activity that was already exiting ignores the sign, because it has PF_EXITING.

6. Impasse. With out manually aborting the FUSE connection, issues will dangle perpetually.

It doesn’t actually make sense to attend for flushes on this case: the duty is dying, so there’s no person to inform the return code of flush() to. It additionally seems that this bug can occur with a number of filesystems (something that calls the kernel’s wait code in flush(), i.e. mainly something that talks to one thing exterior the native kernel).

Particular person filesystems will should be patched within the meantime, for instance the repair for FUSE is here, which was launched on April 23 in Linux 6.3.

Whereas this weblog publish addresses FUSE deadlocks, there are undoubtedly points within the nfs code and elsewhere, which we have now not hit in manufacturing but, however virtually actually will. You may also see it as a symptom of other filesystem bugs. One thing to look out for if in case you have a pid namespace that received’t exit.

That is only a small style of the number of unusual points we encounter operating containers at scale at Netflix. Our workforce is hiring, so please attain out if you happen to additionally love crimson herrings and kernel deadlocks!

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