Re: [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-timequiescent-state race

[Josh Triplett]

On Thu, Aug 30, 2012 at 11:18:31AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> Now the the grace-period initialization procedure is preemptible, it is
> subject to the following race on systems whose rcu_node tree contains
> more than one node:
> 
> 1.	CPU 31 starts initializing the grace period, including the
> 	first leaf rcu_node structures, and is then preempted.
> 
> 2.	CPU 0 refers to the first leaf rcu_node structure, and notes
> 	that a new grace period has started.  It passes through a
> 	quiescent state shortly thereafter, and informs the RCU core
> 	of this rite of passage.
> 
> 3.	CPU 0 enters an RCU read-side critical section, acquiring
> 	a pointer to an RCU-protected data item.
> 
> 4.	CPU 31 removes the data item referenced by CPU 0 from the
> 	data structure, and registers an RCU callback in order to
> 	free it.
> 
> 5.	CPU 31 resumes initializing the grace period, including its
> 	own rcu_node structure.  In invokes rcu_start_gp_per_cpu(),
> 	which advances all callbacks, including the one registered
> 	in #4 above, to be handled by the current grace period.
> 
> 6.	The remaining CPUs pass through quiescent states and inform
> 	the RCU core, but CPU 0 remains in its RCU read-side critical
> 	section, still referencing the now-removed data item.
> 
> 7.	The grace period completes and all the callbacks are invoked,
> 	including the one that frees the data item that CPU 0 is still
> 	referencing.  Oops!!!
> 
> This commit therefore moves the callback handling to precede initialization
> of any of the rcu_node structures, thus avoiding this race.

I don't think it makes sense to introduce and subsequently fix a race in
the same patch series. :)

Could you squash this patch into the one moving grace-period
initialization into a kthread?

- Josh Triplett

> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcutree.c |   33 +++++++++++++++++++--------------
>  1 files changed, 19 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 55f20fd..d435009 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1028,20 +1028,6 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
>  	/* Prior grace period ended, so advance callbacks for current CPU. */
>  	__rcu_process_gp_end(rsp, rnp, rdp);
>  
> -	/*
> -	 * Because this CPU just now started the new grace period, we know
> -	 * that all of its callbacks will be covered by this upcoming grace
> -	 * period, even the ones that were registered arbitrarily recently.
> -	 * Therefore, advance all outstanding callbacks to RCU_WAIT_TAIL.
> -	 *
> -	 * Other CPUs cannot be sure exactly when the grace period started.
> -	 * Therefore, their recently registered callbacks must pass through
> -	 * an additional RCU_NEXT_READY stage, so that they will be handled
> -	 * by the next RCU grace period.
> -	 */
> -	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> -	rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> -
>  	/* Set state so that this CPU will detect the next quiescent state. */
>  	__note_new_gpnum(rsp, rnp, rdp);
>  }
> @@ -1068,6 +1054,25 @@ static int rcu_gp_init(struct rcu_state *rsp)
>  	rsp->gpnum++;
>  	trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
>  	record_gp_stall_check_time(rsp);
> +
> +	/*
> +	 * Because this CPU just now started the new grace period, we
> +	 * know that all of its callbacks will be covered by this upcoming
> +	 * grace period, even the ones that were registered arbitrarily
> +	 * recently.    Therefore, advance all RCU_NEXT_TAIL callbacks
> +	 * to RCU_NEXT_READY_TAIL.  When the CPU later recognizes the
> +	 * start of the new grace period, it will advance all callbacks
> +	 * one position, which will cause all of its current outstanding
> +	 * callbacks to be handled by the newly started grace period.
> +	 *
> +	 * Other CPUs cannot be sure exactly when the grace period started.
> +	 * Therefore, their recently registered callbacks must pass through
> +	 * an additional RCU_NEXT_READY stage, so that they will be handled
> +	 * by the next RCU grace period.
> +	 */
> +	rdp = __this_cpu_ptr(rsp->rda);
> +	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> +
>  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  
>  	/* Exclude any concurrent CPU-hotplug operations. */
> -- 
> 1.7.8
> 
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