On Wed, Jan 09, 2013 at 09:54:11AM -0500, Steven Rostedt wrote:
> On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:
>
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 07912dd..bf4f72d 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -484,7 +484,7 @@ void vtime_task_switch(struct task_struct *prev)
> > * vtime_account().
> > */
> > #ifndef __ARCH_HAS_VTIME_ACCOUNT
> > -void vtime_account(struct task_struct *tsk)
> > +void vtime_account_irq_enter(struct task_struct *tsk)
> > {
> > if (!in_interrupt()) {
> > /*
> > @@ -505,7 +505,7 @@ void vtime_account(struct task_struct *tsk)
> > }
> > vtime_account_system(tsk);
> > }
> > -EXPORT_SYMBOL_GPL(vtime_account);
> > +EXPORT_SYMBOL_GPL(vtime_account_irq_enter);
> > #endif /* __ARCH_HAS_VTIME_ACCOUNT */
> > #endif /* CONFIG_VIRT_CPU_ACCOUNTING */
> >
> > @@ -616,41 +616,67 @@ void thread_group_cputime_adjusted(struct task_struct
> > *p, cputime_t *ut, cputime
> > #endif /* !CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
> >
> > #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> > -static DEFINE_PER_CPU(long, last_jiffies) = INITIAL_JIFFIES;
> > -
> > -static cputime_t get_vtime_delta(void)
> > +static cputime_t get_vtime_delta(struct task_struct *tsk)
> > {
> > long delta;
> >
> > - delta = jiffies - __this_cpu_read(last_jiffies);
> > - __this_cpu_add(last_jiffies, delta);
> > + delta = jiffies - tsk->prev_jiffies;
> > + tsk->prev_jiffies += delta;
> >
> > return jiffies_to_cputime(delta);
> > }
> >
> > -void vtime_account_system(struct task_struct *tsk)
> > +static void __vtime_account_system(struct task_struct *tsk)
> > {
> > - cputime_t delta_cpu = get_vtime_delta();
> > + cputime_t delta_cpu = get_vtime_delta(tsk);
> >
> > account_system_time(tsk, irq_count(), delta_cpu,
> > cputime_to_scaled(delta_cpu));
> > }
> >
> > +void vtime_account_system(struct task_struct *tsk)
> > +{
> > + write_seqlock(&tsk->vtime_seqlock);
> > + __vtime_account_system(tsk);
>
> __vtime_account_system() calls account_system_time()
> account_system_time() calls __account_system_time()
> __account_system_time() calls acct_update_integrals()
> (when CONFIG_TASK_XACCT is set)
> acct_update_integrals() calls task_cputime()
> task_cputime() grabs t->vtime_seqlock for read
>
> DEADLOCK
>
> ironically the subject says *Safely* read cputime ;-)
Well at least it crashes safely. Safely as in "deterministic".
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