linux-kernel - Re: [PATCH 06/33] cputime: Safely read cputime of full dynticks CPUs

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Date:	Wed, 09 Jan 2013 09:54:11 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/33] cputime: Safely read cputime of full dynticks CPUs

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 ;-)

-- Steve

> +	write_sequnlock(&tsk->vtime_seqlock);
> +}
> +
> +void vtime_account_irq_exit(struct task_struct *tsk)
> +{
> +	write_seqlock(&tsk->vtime_seqlock);
> +	if (context_tracking_in_user())
> +		tsk->prev_jiffies_whence = JIFFIES_USER;
> +	__vtime_account_system(tsk);
> +	write_sequnlock(&tsk->vtime_seqlock);
> +}
> +


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