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* bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
@ 2022-07-20 21:39 Ludovic Courtès
  2022-07-20 23:48 ` Maxime Devos
  2022-11-13 23:16 ` Ludovic Courtès
  0 siblings, 2 replies; 10+ messages in thread
From: Ludovic Courtès @ 2022-07-20 21:39 UTC (permalink / raw)
  To: 56674

Hi!

We’ve just had a bad experience with the nginx service on berlin, where
‘herd restart nginx’ would cause shepherd to get stuck forever in
‘waitpid’ on the process that was supposed to start nginx.

The details are unclear, but one thing is clear is that using ‘waitpid’
(either directly or indirectly with ‘system*’, which is what
‘nginx-service-type’ does) is not great:

  1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’
     is in ‘waitpid’ waiting for child process completion (“stuck” as
     in: doesn’t do anything, not even answering ‘herd’ requests or
     inetd connections.)

  2. I don’t think that can happen with ‘system*’ (because it’s in C),
     but generally speaking, there’s a possibility that shepherd’s event
     loop will handle child process termination before some other
     user-made ‘waitpid’ call does.

Anyway, that’s a bad situation.

So I can think of several ways to address it:

  1. Change the nginx service ‘stop’ method to just
     (make-kill-destructor), which should work just as well as invoking
     “nginx -s stop”.

  2. Have Shepherd provide a replacement for ‘system*’.

Thoughts?

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
  2022-07-20 21:39 bug#56674: [Shepherd] Use of ‘waitpid’, ‘system*’, etc. in service code can cause deadlocks Ludovic Courtès
@ 2022-07-20 23:48 ` Maxime Devos
  2022-07-21 15:39   ` Ludovic Courtès
  2022-11-13 23:16 ` Ludovic Courtès
  1 sibling, 1 reply; 10+ messages in thread
From: Maxime Devos @ 2022-07-20 23:48 UTC (permalink / raw)
  To: Ludovic Courtès, 56674


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On 20-07-2022 23:39, Ludovic Courtès wrote:
> Hi!
>
> We’ve just had a bad experience with the nginx service on berlin, where
> ‘herd restart nginx’ would cause shepherd to get stuck forever in
> ‘waitpid’ on the process that was supposed to start nginx.
>
> The details are unclear, but one thing is clear is that using ‘waitpid’
> (either directly or indirectly with ‘system*’, which is what
> ‘nginx-service-type’ does) is not great:
>
>    1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’
>       is in ‘waitpid’ waiting for child process completion (“stuck” as
>       in: doesn’t do anything, not even answering ‘herd’ requests or
>       inetd connections.)
>
>    2. I don’t think that can happen with ‘system*’ (because it’s in C),
>       but generally speaking, there’s a possibility that shepherd’s event
>       loop will handle child process termination before some other
>       user-made ‘waitpid’ call does.
>
> Anyway, that’s a bad situation.
>
> So I can think of several ways to address it:
>
>    1. Change the nginx service ‘stop’ method to just
>       (make-kill-destructor), which should work just as well as invoking
>       “nginx -s stop”.
>
>    2. Have Shepherd provide a replacement for ‘system*’.
Why Shepherd and not guile fibers? Is this a Shepherd-specific problem?
>
> Thoughts?

3. Make waitpid (or a variant that does what we need) interact well with 
guile-fibers, like how 'accept' is doesn't inhibit switching to another 
fiber. There some Linux API with signal handlers or pid fds or such that 
might be useful here, though I don't recall the name. Presumably 
something similar can be done for the Hurd, though some C glue may be 
needed to access the right Hurd APIs if the signal handler API isn't 
portable.

Alternatively:

4. Do the waitpid in a separate thread (needs work-around for the 
multi-threaded fork problem, probably C things? Or modifying Guile and 
maybe glibc to avoid async-unsafe things or make more things async-safe 
or whatever the appropriate ...-safe is here.)

If not a Guile Fibers interaction problem, then the asynchronous signal 
handler API might still be useful.

Greetings,
Maxime


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^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
  2022-07-20 23:48 ` Maxime Devos
@ 2022-07-21 15:39   ` Ludovic Courtès
  2022-08-13 14:59     ` Maxime Devos
  0 siblings, 1 reply; 10+ messages in thread
From: Ludovic Courtès @ 2022-07-21 15:39 UTC (permalink / raw)
  To: Maxime Devos; +Cc: 56674

Maxime Devos <maximedevos@telenet.be> skribis:

> Why Shepherd and not guile fibers? Is this a Shepherd-specific problem?

Blocking calls are a problem for Fibers in general, and ‘waitpid’ is no
exception.

The problem here is Shepherd-specific in the sense that we’re more
likely to use ‘system*’ and ‘waitpid’ in this context.  It’s also
Shepherd-specific because shepherd already runs an event loop that
tracks signal FDs and will thus “see” SIGCHLD events.

> 3. Make waitpid (or a variant that does what we need) interact well
> with guile-fibers, like how 'accept' is doesn't inhibit switching to
> another fiber. There some Linux API with signal handlers or pid fds or
> such that might be useful here, though I don't recall the
> name. Presumably something similar can be done for the Hurd, though
> some C glue may be needed to access the right Hurd APIs if the signal
> handler API isn't portable.

Yes, that’s roughly what I had in mind when I mentioned providing a
replacement for ‘system*’ (but you’re right, it’s a replacement for
‘waitpid’ at its core).

> Alternatively:
>
> 4. Do the waitpid in a separate thread (needs work-around for the
> multi-threaded fork problem, probably C things? Or modifying Guile and
> maybe glibc to avoid async-unsafe things or make more things
> async-safe or whatever the appropriate ...-safe is here.)

For shepherd, multithreading is not an option due to the semantics of
fork in the presence of threads.

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
  2022-07-21 15:39   ` Ludovic Courtès
@ 2022-08-13 14:59     ` Maxime Devos
  0 siblings, 0 replies; 10+ messages in thread
From: Maxime Devos @ 2022-08-13 14:59 UTC (permalink / raw)
  To: Ludovic Courtès; +Cc: 56674


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On 21-07-2022 17:39, Ludovic Courtès wrote:
>> Alternatively:
>>
>> 4. Do the waitpid in a separate thread (needs work-around for the
>> multi-threaded fork problem, probably C things? Or modifying Guile and
>> maybe glibc to avoid async-unsafe things or make more things
>> async-safe or whatever the appropriate ...-safe is here.)
> For shepherd, multithreading is not an option due to the semantics of
> fork in the presence of threads.

 From what I've read, multi-threaded fork is safe as long as you do an 
exec 'immediately' afterwards, without doing things like taking locks or 
allocating memory with malloc in-between the fork and exec. I don't 
think it's possible to do that in Guile code, but that's what the C 
things are for.

Greetings,
Maxime.

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^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#56674: [Shepherd] Use of ‘waitpid’,  ‘system*’,  etc. in service code can cause deadlocks
  2022-07-20 21:39 bug#56674: [Shepherd] Use of ‘waitpid’, ‘system*’, etc. in service code can cause deadlocks Ludovic Courtès
  2022-07-20 23:48 ` Maxime Devos
@ 2022-11-13 23:16 ` Ludovic Courtès
  2022-11-14 16:32   ` bug#58926: Shepherd becomes unresponsive after an interrupt Ludovic Courtès
  1 sibling, 1 reply; 10+ messages in thread
From: Ludovic Courtès @ 2022-11-13 23:16 UTC (permalink / raw)
  To: 56674

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Hi,

Ludovic Courtès <ludo@gnu.org> skribis:

>   1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’
>      is in ‘waitpid’ waiting for child process completion (“stuck” as
>      in: doesn’t do anything, not even answering ‘herd’ requests or
>      inetd connections.)
>
>   2. I don’t think that can happen with ‘system*’ (because it’s in C),
>      but generally speaking, there’s a possibility that shepherd’s event
>      loop will handle child process termination before some other
>      user-made ‘waitpid’ call does.
>
> Anyway, that’s a bad situation.
>
> So I can think of several ways to address it:
>
>   1. Change the nginx service ‘stop’ method to just
>      (make-kill-destructor), which should work just as well as invoking
>      “nginx -s stop”.
>
>   2. Have Shepherd provide a replacement for ‘system*’.

These fresh Shepherd commits install a non-blocking ‘system*’ replacement:

  975b0aa service: Provide a non-blocking replacement of 'system*'.
  039c7a8 service: Spawn a fiber responsible for process monitoring.

We’ll have to do more testing and probably go for a 0.9.3 release soon.

Protip: you can test the latest shepherd with:

--8<---------------cut here---------------start------------->8---
(operating-system
  ;; …
  (essential-services
   (modify-services (operating-system-default-essential-services
                     this-operating-system)
     (shepherd-root-service-type
      config =>
      (shepherd-configuration
       (shepherd (package
                   (inherit shepherd-0.9)
                   (version "0.9.3pre")
                   (source (git-checkout
                            (url "https://git.savannah.gnu.org/git/shepherd.git")))
                   (native-inputs
                    (modify-inputs (package-native-inputs shepherd-0.9)
                      (append autoconf automake help2man texinfo gnu-gettext))))))))))
--8<---------------cut here---------------end--------------->8---

Full example attached.

Ludo’.


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;; This is an operating system configuration template
;; for a "bare bones" setup, with no X11 display server.

(use-modules (gnu) (guix) (guix git))
(use-service-modules networking ssh web vpn shepherd)
(use-package-modules linux screen ssh
                     admin autotools gettext man texinfo)

(operating-system
  (host-name "komputilo")
  (timezone "Europe/Berlin")
  (locale "en_US.utf8")

  ;; Boot in "legacy" BIOS mode, assuming /dev/sdX is the
  ;; target hard disk, and "my-root" is the label of the target
  ;; root file system.
  (bootloader (bootloader-configuration
               (bootloader grub-bootloader)
               (targets '("/dev/sdX"))))
  ;; It's fitting to support the equally bare bones ‘-nographic’
  ;; QEMU option, which also nicely sidesteps forcing QWERTY.
  (kernel-arguments (list "console=ttyS0,115200"))
  (file-systems (cons (file-system
                        (device (file-system-label "my-root"))
                        (mount-point "/")
                        (type "ext4"))
                      %base-file-systems))

  ;; This is where user accounts are specified.  The "root"
  ;; account is implicit, and is initially created with the
  ;; empty password.
  (users (cons (user-account
                (name "alice")
                (comment "Bob's sister")
                (group "users")

                ;; Adding the account to the "wheel" group
                ;; makes it a sudoer.  Adding it to "audio"
                ;; and "video" allows the user to play sound
                ;; and access the webcam.
                (supplementary-groups '("wheel"
                                        "audio" "video")))
               %base-user-accounts))

  ;; Globally-installed packages.
  (packages (append (list screen strace) %base-packages))

  (essential-services
   (modify-services (operating-system-default-essential-services
                     this-operating-system)
     (shepherd-root-service-type
      config =>
      (shepherd-configuration
       (shepherd (package
                   (inherit shepherd-0.9)
                   (version "0.9.3pre")
                   (source (git-checkout
                            (url "https://git.savannah.gnu.org/git/shepherd.git")))
                   (native-inputs
                    (modify-inputs (package-native-inputs shepherd-0.9)
                      (append autoconf automake help2man texinfo gnu-gettext)))))))))

  ;; Add services to the baseline: a DHCP client and
  ;; an SSH server.
  (services (append (list (service dhcp-client-service-type)
                          (service nginx-service-type
                                   (nginx-configuration
                                    (server-blocks
                                     (list (nginx-server-configuration
                                            (listen '("80"))
                                            (server-name '("www.example.org"))
                                            (root "/srv/whatever"))))))
                          (service wireguard-service-type
                                   (wireguard-configuration
                                    (addresses (list "10.0.0.2/24"))
                                    (dns '("10.0.0.50")))) ;does not exit


                          (service openssh-service-type
                                   (openssh-configuration
                                    (openssh openssh-sans-x)
                                    (port-number 2222))))
                    %base-services)))

^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#58926: Shepherd becomes unresponsive after an interrupt
  2022-11-13 23:16 ` Ludovic Courtès
@ 2022-11-14 16:32   ` Ludovic Courtès
  0 siblings, 0 replies; 10+ messages in thread
From: Ludovic Courtès @ 2022-11-14 16:32 UTC (permalink / raw)
  To: 56674; +Cc: Mathieu Othacehe, 58926

Hello!

Ludovic Courtès <ludo@gnu.org> skribis:

> These fresh Shepherd commits install a non-blocking ‘system*’ replacement:
>
>   975b0aa service: Provide a non-blocking replacement of 'system*'.
>   039c7a8 service: Spawn a fiber responsible for process monitoring.
>
> We’ll have to do more testing and probably go for a 0.9.3 release soon.

Shepherd commit ada88074f0ab7551fd0f3dce8bf06de971382e79 passes my
tests.  It definitely solves the wireguard example and similar things
(uses of ‘system*’ in service constructors/destructors); I can’t tell
for sure about nginx because I haven’t been able to reproduce it in a
VM.  I’m interested in ways to reproduce it.

It does look like we could go with 0.9.3 real soon now.

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#58926: Shepherd becomes unresponsive after an interrupt
  2022-10-31 12:44 Mathieu Othacehe
  2022-11-10  9:59 ` Ludovic Courtès
  2022-11-12 18:10 ` Ludovic Courtès
@ 2022-11-12 18:28 ` Ludovic Courtès
  2 siblings, 0 replies; 10+ messages in thread
From: Ludovic Courtès @ 2022-11-12 18:28 UTC (permalink / raw)
  To: Mathieu Othacehe; +Cc: 58926

Mathieu Othacehe <othacehe@gnu.org> skribis:

> then hitting C-c, Shepherd becomes totally unresponsive:
>
> sudo herd status
>
>
> and all further Shpeherd commands hang forever. I was able to reproduce
> it in two different configurations:

[...]

> 2. On Berlin, while trying to restart nginx.

I can’t reproduce it in a VM.

Before I try it on a production system :-), does anyone have a tip on
how to reproduce it?  Or perhaps strace output from a system that
exhibits this bug?

TIA!

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#58926: Shepherd becomes unresponsive after an interrupt
  2022-10-31 12:44 Mathieu Othacehe
  2022-11-10  9:59 ` Ludovic Courtès
@ 2022-11-12 18:10 ` Ludovic Courtès
  2022-11-12 18:28 ` Ludovic Courtès
  2 siblings, 0 replies; 10+ messages in thread
From: Ludovic Courtès @ 2022-11-12 18:10 UTC (permalink / raw)
  To: Mathieu Othacehe; +Cc: 53225, 58926

Mathieu Othacehe <othacehe@gnu.org> skribis:

> 1. On my laptop with a Wireguard service trying to reach a non-existing
> DNS server.
>
>             (service wireguard-service-type
>                      (wireguard-configuration
>                       (addresses (list "10.0.0.2/24"))
>                       (dns '("10.0.0.50")) #does not exit

This one is similar to:

  https://issues.guix.gnu.org/53225
  https://issues.guix.gnu.org/53381

It has to do with the fact that “wg-quick up” blocks until it succeeds
and that ‘invoke’ gets stuck on ‘waitpid’ until the “wg-quick” process
terminates.

The solution will be to use something non-blocking instead of ‘invoke’;
I’m looking into it.

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#58926: Shepherd becomes unresponsive after an interrupt
  2022-10-31 12:44 Mathieu Othacehe
@ 2022-11-10  9:59 ` Ludovic Courtès
  2022-11-12 18:10 ` Ludovic Courtès
  2022-11-12 18:28 ` Ludovic Courtès
  2 siblings, 0 replies; 10+ messages in thread
From: Ludovic Courtès @ 2022-11-10  9:59 UTC (permalink / raw)
  To: Mathieu Othacehe; +Cc: 58926

Hi,

Mathieu Othacehe <othacehe@gnu.org> skribis:

> sudo herd restart service-that-hangs-upon-restart
>
>
> then hitting C-c, Shepherd becomes totally unresponsive:
>
> sudo herd status
>
>
> and all further Shpeherd commands hang forever. I was able to reproduce
> it in two different configurations:
>
> 1. On my laptop with a Wireguard service trying to reach a non-existing
> DNS server.
>
>             (service wireguard-service-type
>                      (wireguard-configuration
>                       (addresses (list "10.0.0.2/24"))
>                       (dns '("10.0.0.50")) #does not exit
>
> 2. On Berlin, while trying to restart nginx.

I experienced case #2: in that case ‘strace -p1’ showed that shepherd
was stuck on waitpid of the nginx process, which was not terminating.
Killing that process would unlock shepherd.

This might be <https://issues.guix.gnu.org/56674>.

Would be good to see what’s up with WireGuard.

Ludo’.




^ permalink raw reply	[flat|nested] 10+ messages in thread

* bug#58926: Shepherd becomes unresponsive after an interrupt
@ 2022-10-31 12:44 Mathieu Othacehe
  2022-11-10  9:59 ` Ludovic Courtès
                   ` (2 more replies)
  0 siblings, 3 replies; 10+ messages in thread
From: Mathieu Othacehe @ 2022-10-31 12:44 UTC (permalink / raw)
  To: 58926


Hello,

When running the following command:

--8<---------------cut here---------------start------------->8---
sudo herd restart service-that-hangs-upon-restart
--8<---------------cut here---------------end--------------->8---

then hitting C-c, Shepherd becomes totally unresponsive:

--8<---------------cut here---------------start------------->8---
sudo herd status
--8<---------------cut here---------------end--------------->8---

and all further Shpeherd commands hang forever. I was able to reproduce
it in two different configurations:

1. On my laptop with a Wireguard service trying to reach a non-existing
DNS server.

--8<---------------cut here---------------start------------->8---
            (service wireguard-service-type
                     (wireguard-configuration
                      (addresses (list "10.0.0.2/24"))
                      (dns '("10.0.0.50")) #does not exit
--8<---------------cut here---------------end--------------->8---

2. On Berlin, while trying to restart nginx.

In both situations, the "reboot" command was also hanging.

Thanks,

Mathieu




^ permalink raw reply	[flat|nested] 10+ messages in thread

end of thread, other threads:[~2022-11-14 23:47 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-07-20 21:39 bug#56674: [Shepherd] Use of ‘waitpid’, ‘system*’, etc. in service code can cause deadlocks Ludovic Courtès
2022-07-20 23:48 ` Maxime Devos
2022-07-21 15:39   ` Ludovic Courtès
2022-08-13 14:59     ` Maxime Devos
2022-11-13 23:16 ` Ludovic Courtès
2022-11-14 16:32   ` bug#58926: Shepherd becomes unresponsive after an interrupt Ludovic Courtès
2022-10-31 12:44 Mathieu Othacehe
2022-11-10  9:59 ` Ludovic Courtès
2022-11-12 18:10 ` Ludovic Courtès
2022-11-12 18:28 ` Ludovic Courtès

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