Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID

[Christian Brauner]

On Fri, Aug 02, 2019 at 03:30:01PM +0200, Oleg Nesterov wrote:
> On 08/02, Christian Brauner wrote:
> >
> > On Wed, Jul 31, 2019 at 06:12:22PM +0200, Adrian Reber wrote:
> > > The main motivation to add CLONE_SET_TID to clone3() is CRIU.
> > >
> > > To restore a process with the same PID/TID CRIU currently uses
> > > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > > ns_last_pid and then (quickly) does a clone(). This works most of the
> > > time, but it is racy. It is also slow as it requires multiple syscalls.
> >
> > Can you elaborate how this is racy, please. Afaict, CRIU will always
> > usually restore in a new pid namespace that it controls, right?
> 
> Why? No. For example you can checkpoint (not sure this is correct word)
> a single process in your namespace, then (try to restore) it. 
> 
> > What is
> > the exact race?
> 
> something else in the same namespace can fork() right after criu writes
> the pid-for-restore into ns_last_pid.

Ok, that makes sense. :)
My CRIU userspace knowledge is sporadic, so I'm not sure how exactly it
restores process trees in pid namespaces and what workloads this would
especially help with.