linux-kernel - Re: [PATCH 3/8] posix-cpu-timers: fix wrong timer initialization

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Date:	Tue, 18 Jun 2013 11:12:17 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Olivier Langlois <olivier@...llion01.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/8] posix-cpu-timers: fix wrong timer initialization

On Tue, Jun 18, 2013 at 10:20 AM, Frederic Weisbecker
<fweisbec@...il.com> wrote:
> On Sun, May 26, 2013 at 05:35:44PM -0400, kosaki.motohiro@...il.com wrote:
>> From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>>
>> Currently glibc's rt/tst-cputimer1 testcase sporadically fails because
>> a timer created by timer_create() may fire earlier than specified.
>>
>> posix_cpu_timer_set() uses "val" as current time for three purpose. 1)
>> initialize sig->cputimer. 2) calculation "old" val. 3) calculations an
>> expires.
>>
>> (1) and (2) should only use committed time (i.e. without delta_exec)
>> because run_posix_cpu_timers() don't care of delta_exec and we need
>> consistency, but (3) need exact current time (aka cpu clock time) because
>> an expires should be "now + timeout" by definition.
>>
>> This patch distinguishes between two kinds of "now".
>>
>> Cc: Olivier Langlois <olivier@...llion01.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Frederic Weisbecker <fweisbec@...il.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Acked-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>> ---
>>  include/linux/kernel_stat.h |    5 -----
>>  kernel/posix-cpu-timers.c   |   14 ++++++++++++--
>>  kernel/sched/core.c         |   13 -------------
>>  3 files changed, 12 insertions(+), 20 deletions(-)
>>
>> diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
>> index ed5f6ed..f5d4fdf 100644
>> --- a/include/linux/kernel_stat.h
>> +++ b/include/linux/kernel_stat.h
>> @@ -117,11 +117,6 @@ static inline unsigned int kstat_cpu_irqs_sum(unsigned int cpu)
>>       return kstat_cpu(cpu).irqs_sum;
>>  }
>>
>> -/*
>> - * Lock/unlock the current runqueue - to extract task statistics:
>> - */
>> -extern unsigned long long task_delta_exec(struct task_struct *);
>> -
>>  extern void account_user_time(struct task_struct *, cputime_t, cputime_t);
>>  extern void account_system_time(struct task_struct *, int, cputime_t, cputime_t);
>>  extern void account_steal_time(cputime_t);
>> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
>> index 25447c5..d068808 100644
>> --- a/kernel/posix-cpu-timers.c
>> +++ b/kernel/posix-cpu-timers.c
>> @@ -652,7 +652,7 @@ static int cpu_timer_sample_group(const clockid_t which_clock,
>>               cpu->cpu = cputime.utime;
>>               break;
>>       case CPUCLOCK_SCHED:
>> -             cpu->sched = cputime.sum_exec_runtime + task_delta_exec(p);
>> +             cpu->sched = cputime.sum_exec_runtime;
>
> Are you sure that all callers of cpu_timer_sample_group() are fine with that change?

Now, cpu_timer_sample_group() is used from following four points.

posix_cpu_timer_set(): for timer initialization
posix_cpu_timer_get(): for timer_gettime(2)
posix_cpu_timer_schedule(): timer firing
set_process_cpu_timer(): for itimer

I think all of them are safe because, the point is, timer firing
procedure (check_thread_timers and check_process_timers) don't care
uncommitted delta. Then, other timer functions need to use the same
timer tick. Otherwise the inconsistency leak to userland sooner or
later.

The another solution is, check_{thread/process}_timers take plenty rq
locks and use accurate time. However, of course, it may make lots
performance hit. So, I don't want
to take this way.


> Looking at set_process_cpu_timer() it seems we want the committed time as well to
> be added on newval. For the same reasons we use cpu_clock_sample_group() in (3) here.

Sorry, I haven't caught your point. Could you elaborate more?
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