Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

[Manfred Spraul <manfred@colorfullife.com>]

Lai Jiangshan wrote:
> I just had a fast review. so my comments is nothing but cleanup.
>
>           Thanks, Lai.
>
> Paul E. McKenney wrote:
>   
>> Hello!
>>     
>
>   
>> +rcu_start_gp(struct rcu_state *rsp, unsigned long iflg)
>> +	__releases(rsp->rda[smp_processor_id()]->lock)
>> +{
>> +	unsigned long flags = iflg;
>> +	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
>> +	struct rcu_node *rnp = rcu_get_root(rsp);
>> +	struct rcu_node *rnp_cur;
>> +	struct rcu_node *rnp_end;
>> +
>> +	if (!cpu_needs_another_gp(rsp, rdp)) {
>>  
>>  		/*
>> -		 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
>> -		 * Barrier  Otherwise it can cause tickless idle CPUs to be
>> -		 * included in rcp->cpumask, which will extend graceperiods
>> -		 * unnecessarily.
>> +		 * Either there is no need to detect any more grace periods
>> +		 * at the moment, or we are already in the process of
>> +		 * detecting one.  Either way, we should not start a new
>> +		 * RCU grace period, so drop the lock and return.
>>  		 */
>> -		smp_mb();
>> -		cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
>> +		spin_unlock_irqrestore(&rnp->lock, flags);
>> +		return;
>> +	}
>> +
>> +	/* Advance to a new grace period and initialize state. */
>> +
>> +	rsp->gpnum++;
>> +	rsp->signaled = RCU_SIGNAL_INIT;
>> +	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
>> +	record_gp_stall_check_time();
>> +	dyntick_save_completed(rsp, rsp->completed - 1);
>> +	note_new_gpnum(rsp, rdp);
>> +
>> +	/*
>> +	 * Because we are first, we know that all our callbacks will
>> +	 * be covered by this upcoming grace period, even the ones
>> +	 * that were registered arbitrarily recently.
>> +	 */
>> +
>> +	rcu_next_callbacks_are_ready(rdp);
>> +	rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
>>  
>> -		rcp->signaled = 0;
>> +	/* Special-case the common single-level case. */
>> +
>> +	if (NUM_RCU_NODES == 1) {
>> +		rnp->qsmask = rnp->qsmaskinit;
>>     
>
> I tried a mask like qsmaskinit before. The system came to deadlock
> when I did on/offline cpus.
> I didn't find out the whys for I bethought of these two problem:
>
> problem 1:
> ----race condition 1:
> <cpu_down>
> synchronize_rcu <called from offline handler in other subsystem>
> rcu_offline_cpu
>
>
> -----race condition 2:
> rcu_online_cpu
> synchronize_rcu <called from online handler in other subsystem>
> <cpu_up>
>
> in these two condition, synchronize_rcu isblocked for ever for
> synchronize_rcu have to wait a cpu in rnp->qsmask, but this
> cpu don't run.
>
>   
Can we disallow synchronize_rcu() from the cpu notifiers? Are there any 
users that do a synchronize_rcu() from within the notifiers?
I don't see any other solution.
Something like qsmaskinit is needed - always enumerating all cpus just 
doesn't scale.

Perhaps it's possible to rely on CPU_DYING, but I haven't figured out 
yet how to handle read-side critical sections in CPU_DYING handlers.
Interrupts after CPU_DYING could be handled by rcu_irq_enter(), 
rcu_irq_exit() [yes, they exist on x86: the arch code enables the local 
interrupts in order to process the currently queued interrupts]

--
    Manfred