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    SubjectRe: [PATCH resend] Reading POSIX CPU timer from outside the process.
    On 12/28, Dario Faggioli wrote:
    >
    > On Tue, 2010-12-28 at 17:38 +0100, Oleg Nesterov wrote:
    >
    > > > rcu_read_lock();
    > > > p = find_task_by_vpid(pid);
    > > > - if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
    > > > - same_thread_group(p, current) : has_group_leader_pid(p))) {
    > > > + if (!p || (CPUCLOCK_PERTHREAD(which_clock) &&
    > > > + same_thread_group(p, current) && !has_group_leader_pid(p)))
    > > > error = -EINVAL;
    > > > - }
    > > > rcu_read_unlock();
    > >
    > > How so? For example, with this change
    > > clock_getres(MAKE_THREAD_CPUCLOCK(pid_of_sub_thread)) won't work, no?
    > >
    > I tested all the clock_getres() calls that came to my mind (at least the
    > one that are possible from an userspace program), and they always worked
    > because of this (still in check_clock):
    >
    > const pid_t pid = CPUCLOCK_PID(which_clock);
    >
    > if (pid == 0)
    > return 0;
    >
    > Which triggers all the times,

    No, please note pid_of_sub_thread above.

    > The whole point was about, given the current implementation of
    > clock_getcpuclockid done by glibc, can we remove that "failed with
    > success" (showed in the changelog) thing and come up with some
    > meaningful clockid for that situation? It's more than possible for the
    > answer to be no!!! :-P

    I think we should change glibc if clock_getcpuclockid() doesn't work,
    please see below.

    > > I think, if we want to remove this limitation, we need something
    > > like the patch below. If it doesn't help, we should fix glibc.
    > >
    > > --- x/kernel/posix-cpu-timers.c
    > > +++ x/kernel/posix-cpu-timers.c
    > > @@ -39,10 +39,8 @@ static int check_clock(const clockid_t w
    > >
    > > rcu_read_lock();
    > > p = find_task_by_vpid(pid);
    > > - if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
    > > - same_thread_group(p, current) : has_group_leader_pid(p))) {
    > > + if (!p || !(CPUCLOCK_PERTHREAD(which_clock) || has_group_leader_pid(p)))
    > > error = -EINVAL;
    > > - }
    > > rcu_read_unlock();
    > >
    > Which won't work because CPUCLOCK_PERTHREAD(which_clock) is always false
    > in this case.

    I guess, this is because glibc passes MAKE_PROCESS_CLOCK() id, right?
    But we shouldn't add the hacks to the kernel to hide the limitations
    in glibc.

    > BTW, again, I see your point, the fix might need to happen at glibc
    > level. I'll check that and come back if I find something interesting.

    Yes, please.


    BTW. What is the test-case? I am looking at http://gitorious.org/clockid,
    I guess it is clockid.c...

    You do not need clock_getcpuclockid() at all. In fact I do not really
    understand what this helper should actually do, probably it is only
    needed to validate the pid. You can simply use MAKE_THREAD_CPUCLOCK()
    to sample a single thread via clock_gettime().

    IOW. Unless I missed something, with this patch, the only problem
    is that getcpuclockid() always assumes MAKE_PROCESS_CPUCLOCK(),
    I do not think this is the kernel problem.

    Oleg.



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