[CRIU] Does CRIU handle Futex system calls ?

[Dorian Goepp]

Hello, 

Thanks a lot Alex for your response. 

There is one thing I would just like to be sure to have understood
properly from your message. Do you mean that the process will redo the
futex_wait system calls on restoration ? 

Best regards, 

Dorian Goepp 

Le 2022-07-07 14:34, Alexander Mikhalitsyn a écrit :

> 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 [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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Links:
------
[1] https://man7.org/linux/man-pages/man2/futex.2.html
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