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

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