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


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