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From: Josh Triplett <josh@joshtriplett.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
	laijs@cn.fujitsu.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
	niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
	dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
	fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org,
	"Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: [PATCH tip/core/rcu 06/15] rcu: Make offline-CPU checking allow for indefinite delays
Date: Fri, 31 Aug 2012 11:12:37 -0700	[thread overview]
Message-ID: <20120831181237.GD4259@jtriplet-mobl1> (raw)
In-Reply-To: <1346352988-32444-6-git-send-email-paulmck@linux.vnet.ibm.com>

On Thu, Aug 30, 2012 at 11:56:19AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> 
> The rcu_implicit_offline_qs() function implicitly assumed that execution
> would progress predictably when interrupts are disabled, which is of course
> not guaranteed when running on a hypervisor.  Furthermore, this function
> is short, and is called from one place only in a short function.
> 
> This commit therefore ensures that the timing is checked before
> checking the condition, which guarantees correct behavior even given
> indefinite delays.  It also inlines rcu_implicit_offline_qs() into
> rcu_implicit_dynticks_qs().
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> ---
>  kernel/rcutree.c |   53 +++++++++++++++++++++--------------------------------
>  1 files changed, 21 insertions(+), 32 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 96b8aff..9f44749 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -317,35 +317,6 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
>  }
>  
>  /*
> - * If the specified CPU is offline, tell the caller that it is in
> - * a quiescent state.  Otherwise, whack it with a reschedule IPI.
> - * Grace periods can end up waiting on an offline CPU when that
> - * CPU is in the process of coming online -- it will be added to the
> - * rcu_node bitmasks before it actually makes it online.  The same thing
> - * can happen while a CPU is in the process of coming online.  Because this
> - * race is quite rare, we check for it after detecting that the grace
> - * period has been delayed rather than checking each and every CPU
> - * each and every time we start a new grace period.
> - */
> -static int rcu_implicit_offline_qs(struct rcu_data *rdp)
> -{
> -	/*
> -	 * If the CPU is offline for more than a jiffy, it is in a quiescent
> -	 * state.  We can trust its state not to change because interrupts
> -	 * are disabled.  The reason for the jiffy's worth of slack is to
> -	 * handle CPUs initializing on the way up and finding their way
> -	 * to the idle loop on the way down.
> -	 */
> -	if (cpu_is_offline(rdp->cpu) &&
> -	    ULONG_CMP_LT(rdp->rsp->gp_start + 2, jiffies)) {
> -		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
> -		rdp->offline_fqs++;
> -		return 1;
> -	}
> -	return 0;
> -}
> -
> -/*
>   * rcu_idle_enter_common - inform RCU that current CPU is moving towards idle
>   *
>   * If the new value of the ->dynticks_nesting counter now is zero,
> @@ -675,7 +646,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
>   * Return true if the specified CPU has passed through a quiescent
>   * state by virtue of being in or having passed through an dynticks
>   * idle state since the last call to dyntick_save_progress_counter()
> - * for this same CPU.
> + * for this same CPU, or by virtue of having been offline.
>   */
>  static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  {
> @@ -699,8 +670,26 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  		return 1;
>  	}
>  
> -	/* Go check for the CPU being offline. */
> -	return rcu_implicit_offline_qs(rdp);
> +	/*
> +	 * Check for the CPU being offline, but only if the grace period
> +	 * is old enough.  We don't need to worry about the CPU changing
> +	 * state: If we see it offline even once, it has been through a
> +	 * quiescent state.
> +	 *
> +	 * The reason for insisting that the grace period be at least
> +	 * one jiffy old is that CPUs that are not quite online and that
> +	 * have just gone offline can still execute RCU read-side critical
> +	 * sections.
> +	 */
> +	if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
> +		return 0;  /* Grace period is not old enough. */
> +	barrier();
> +	if (cpu_is_offline(rdp->cpu)) {
> +		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
> +		rdp->offline_fqs++;
> +		return 1;
> +	}
> +	return 0;
>  }
>  
>  static int jiffies_till_stall_check(void)
> -- 
> 1.7.8
> 

  reply	other threads:[~2012-08-31 18:12 UTC|newest]

Thread overview: 86+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-30 18:56 [PATCH tip/core/rcu 0/15] Miscellaneous fixes Paul E. McKenney
2012-08-30 18:56 ` [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 02/15] rcu: Pull TINY_RCU dyntick-idle tracing into non-idle region Paul E. McKenney
2012-08-31 16:53     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 03/15] rcu: Properly initialize ->boost_tasks on CPU offline Paul E. McKenney
2012-08-31 17:56     ` Josh Triplett
2012-09-06 14:40     ` Peter Zijlstra
2012-09-06 20:58       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context Paul E. McKenney
2012-08-31 18:00     ` Josh Triplett
2012-09-04 22:33       ` Paul E. McKenney
2012-09-04 22:48         ` Josh Triplett
2012-09-04 22:51         ` Steven Rostedt
2012-09-04 23:08           ` Josh Triplett
2012-09-04 23:23             ` Steven Rostedt
2012-09-04 23:33               ` Josh Triplett
2012-09-04 23:43                 ` Paul E. McKenney
2012-09-06 18:54                   ` Josh Triplett
2012-09-06 19:54                     ` Steven Rostedt
2012-09-07  6:09                       ` Josh Triplett
2012-09-07 14:24                         ` Paul E. McKenney
2012-09-07 14:47                           ` Josh Triplett
2012-09-07 15:16                             ` Steven Rostedt
2012-09-12  1:07                               ` Paul E. McKenney
2012-09-12 14:13                                 ` Steven Rostedt
2012-09-12 15:03                                   ` Paul E. McKenney
2012-09-12 15:18                                     ` Steven Rostedt
2012-09-12 16:57                                       ` Paul E. McKenney
2012-09-04 23:46                 ` Steven Rostedt
2012-09-05  0:42                   ` Josh Triplett
2012-09-05  6:23                   ` [PATCH] trace: Don't declare trace_*_rcuidle functions in modules Josh Triplett
2012-09-05 14:26                     ` Mathieu Desnoyers
2012-09-05 16:36                     ` Paul E. McKenney
2012-09-06 19:49                     ` Steven Rostedt
2012-09-14  6:07                     ` [tip:core/rcu] trace: Don' t " tip-bot for Josh Triplett
2012-09-04 23:14           ` [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 05/15] rcu: Improve boost selection when moving tasks to root rcu_node Paul E. McKenney
2012-08-31 18:09     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 06/15] rcu: Make offline-CPU checking allow for indefinite delays Paul E. McKenney
2012-08-31 18:12     ` Josh Triplett [this message]
2012-08-30 18:56   ` [PATCH tip/core/rcu 07/15] rcu: Fix obsolete rcu_initiate_boost() header comment Paul E. McKenney
2012-08-31 18:13     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 08/15] rcu: Apply for_each_rcu_flavor() to increment_cpu_stall_ticks() Paul E. McKenney
2012-08-31 18:15     ` Josh Triplett
2012-09-04 22:44       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault Paul E. McKenney
2012-08-31 18:19     ` Josh Triplett
2012-09-04 22:46       ` Paul E. McKenney
2012-09-04 22:55         ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during CPU stall warnings Paul E. McKenney
2012-08-31 18:23     ` Josh Triplett
2012-09-04 22:51       ` Paul E. McKenney
2012-09-06 14:51     ` Peter Zijlstra
2012-09-06 21:01       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU " Paul E. McKenney
2012-08-31 18:24     ` Josh Triplett
2012-09-06 14:56     ` Peter Zijlstra
2012-09-06 15:07       ` Steven Rostedt
2012-09-06 15:19         ` Peter Zijlstra
2012-09-06 21:03           ` Paul E. McKenney
2012-09-06 21:41             ` Steven Rostedt
2012-09-06 21:58               ` Paul E. McKenney
2012-09-06 22:05                 ` Steven Rostedt
2012-09-06 22:22                   ` Paul E. McKenney
2012-09-07  7:00                     ` Peter Zijlstra
2012-09-07 14:42                       ` Steven Rostedt
2012-08-30 18:56   ` [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier from __call_rcu() Paul E. McKenney
2012-08-31 18:30     ` Josh Triplett
2012-08-31 18:40       ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 13/15] rcu: Move TINY_PREEMPT_RCU away from raw_local_irq_save() Paul E. McKenney
2012-08-31 18:34     ` Josh Triplett
2012-09-04 23:03       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 14/15] time: RCU permitted to stop idle entry via softirq Paul E. McKenney
2012-08-31 18:51     ` Josh Triplett
2012-09-06 15:12     ` Peter Zijlstra
2012-09-06 21:35       ` Paul E. McKenney
2012-09-06 21:57         ` Steven Rostedt
2012-09-06 22:11           ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 15/15] kmemleak: Replace list_for_each_continue_rcu with new interface Paul E. McKenney
2012-08-31 18:55     ` Josh Triplett
2012-09-04 23:41       ` Paul E. McKenney
2012-08-31 16:49   ` [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races Josh Triplett
2012-09-04 22:36     ` Paul E. McKenney
2012-09-06 14:38   ` Peter Zijlstra
2012-09-06 20:51     ` Paul E. McKenney
2012-09-07  6:54       ` Peter Zijlstra

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