[CRIU] Does CRIU handle Futex system calls ?

[Alexander Mikhalitsyn]

Hello, dear friends,

Yep, CRIU definitely handles futexes carefully. It's one of the most
important things.
We support both robust and regular futexes.

As far as you know futexes work only on a shared memory basis, so for
non-robust futex
there is no need to have any special handling. We are just dumping the
whole process memory contents.
So after the restore we just need to ensure that threads instruction
pointers (IP) are properly set
(for instance we need to perform manual "syscall restart" for futexes
(see compel_get_task_regs()).

For robust futexes we have a special handling here:
https://github.com/checkpoint-restore/criu/blob/c8f9880adab038481f7806173b698fc6e17ba76a/criu/cr-dump.c#L565
https://github.com/checkpoint-restore/criu/blob/c8f9880adab038481f7806173b698fc6e17ba76a/criu/pie/restorer.c#L532

Regards,
Alex

On Thu, Jul 7, 2022 at 3:13 PM Adrian Reber <adrian at lisas.de> wrote:
>
> Please try to submit your question as a github issue. Much higher
> chances of getting an answer there.
>
>                 Adrian
>
> On Thu, Jul 07, 2022 at 01:39:17PM +0200, Dorian Goepp wrote:
> > Hi,
> >
> > Is this the right place to ask questions about CRIU's features and
> > internals?
> >
> > If so, I have been considering CRIU for dumping and restoring a process
> > with a set of threads synchronised through futexes [1]. It seems to
> > work, but, I cannot tell from the documentation (wiki) whether it is
> > officially supported, or just accidentally works for the cases I tested
> > it with.
> >
> > I could not find which parts would take care of the state of the
> > futex_wait system call in CRIU's source code, except maybe for
> > `get_task_regs()` in crui/compel/arch/x86/src/lib/infect.c (I run it on
> > an amd64 processor). This function issues the warning "Will restore %d
> > with interrupted system call" when I dump the futex-heavy process. Is it
> > enough to save the process's register state to resume the futex system
> > call correctly ?
> >
> > Best regards,
> >
> > Dorian Goepp
> >
> > Links:
> > ------
> > [1] https://man7.org/linux/man-pages/man2/futex.2.html
>
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