linux-kernel - Re: [PATCH resend] Reading POSIX CPU timer from outside the process.

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Date:	Wed, 29 Dec 2010 14:21:30 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dario Faggioli <raistlin@...ux.it>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	torbenh <torbenh@....de>, john.stultz@...aro.org,
	roland@...hat.com, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: Re: [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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