Messages in this thread | | | Date | Thu, 30 Dec 2010 18:45:29 +0100 | From | torbenh <> | Subject | Re: [PATCH resend] Reading POSIX CPU timer from outside the process. |
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On Wed, Dec 29, 2010 at 02:21:30PM +0100, Oleg Nesterov wrote: > 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.
http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_getcpuclockid.3.html
this one works. ok... it takes a pthread_t for identifying the thread. but it works.
> > > > 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. >
-- torben Hohn
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