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    SubjectRe: + remove-the-likelypid-check-in-copy_process.patch added to -mm tree
    On 03/17, Eric W. Biederman wrote:
    >
    > Oleg Nesterov <oleg@tv-sign.ru> writes:
    >
    > > --- a/init/main.c~explicitly-set-pgid-and-sid-of-init-process
    > > +++ a/init/main.c
    > > @@ -783,6 +783,7 @@ static int __init init(void * unused)
    > > */
    > > init_pid_ns.child_reaper = current;
    > >
    > > + __set_special_pids(1, 1);
    > > cad_pid = task_pid(current);
    > >
    > > smp_prepare_cpus(max_cpus);
    > >
    > > Nice changelog :)
    > >
    > > The patch looks good, except __set_special_pids(1, 1) should be no-op.
    > > This is a child forked by swapper. copy_process() was changed by
    > > use-task_pgrp-task_session-in-copy_process.patch
    > > , but signal->{pgrp,_session} get its value from INIT_SIGNALS ?
    > >
    > > Could you explain this as well? Some other changes I missed?
    >
    > As I recall the patch series started with modifying attach_pid
    > to take a struct pid pointer instead of a pid_t value. It means
    > fewer hash table looks ups and it should help in implementing the pid
    > namespace.
    >
    > Well the initial kernel process does not have a struct pid so when
    > it's children start doing:
    > attach_pid(p, PIDTYPE_PGID, task_group(p));
    > attach_pid(p, PIDTYPE_SID, task_session(p));
    > We will get an oops.

    So far this is the only reason to have init_struct_pid. Because the
    boot CPU (swapper) forks, right?

    > So a dummy unhashed struct pid was added for the idle threads.
    > Allowing several special cases in the code to be removed.
    >
    > With that chance the previous special case to force the idle thread
    > init session 1 pgrp 1 no longer works because attach_pid no longer
    > looks at the pid value but instead at the struct pid pointers.
    >
    > So we had to add the __set_special_pids() to continue to keep init
    > in session 1 pgrp 1. Since /sbin/init calls setsid() that our setting
    > the sid and the pgrp may not be strictly necessary. Still is better
    > to not take any chances.

    Yes, yes, I see. But my (very unclear, sorry) question was: shouldn't we
    change INIT_SIGNALS then? /sbin/init inherits ->pgrp == ->_session == 1,
    in that case __set_special_pids(1,1) does nothing.

    > Anyway the point of removing the likely(pid) check was that it didn't
    > look necessary any longer. But as you have correctly pointed putting
    > it on the task list and incrementing the process count for the idle
    > threads is probably still a problem.

    Yes. Note also that the parent doing fork_idle() is not always swapper,
    it is just wrong to do attach_pid(PIDTYPE_PGID/PIDTYPE_SID) in this case.
    example: arch/x86_64/kernel/smpboot.c:do_boot_cpu()

    > So while we are much better we
    > still have some use for the if (likely(p->pid)) special case.

    Yes, I think this change should be dropped for now.

    > Is that enough to bring you up to speed?

    Thanks for your explanations!

    Oleg.

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