Re: [PATCH tip/core/rcu 06/15] rcu: Make offline-CPU checking allow for indefinite delays

[Josh Triplett <josh@joshtriplett.org>]

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
>