Re: [PATCH v5 09/45] smp, cpu hotplug: Fix smp_call_function_*() toprevent CPU offline properly

[Paul E. McKenney]

On Mon, Feb 11, 2013 at 01:11:29AM +0530, Srivatsa S. Bhat wrote:
> On 02/09/2013 05:37 AM, Paul E. McKenney wrote:
> > On Tue, Jan 22, 2013 at 01:05:10PM +0530, Srivatsa S. Bhat wrote:
> >> Once stop_machine() is gone from the CPU offline path, we won't be able to
> >> depend on preempt_disable() to prevent CPUs from going offline from under us.
> >>
> >> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline,
> >> while invoking from atomic context.
> >>
> >> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> > 
> > Would it make sense for get_online_cpus_atomic() to return the current
> > CPU number?
> 
> Hmm, I'm not so sure. I tried to model it after get_online_cpus(), which doesn't
> return anything (for other reasons, of course..)
> 
> Moreover, a function name like *_cpu_* returning the CPU number would be intuitive.
> But a name such as *_cpus_* (plural) returning a CPU number might appear confusing..
> 
> And also I don't think it'll make a huge improvement in the callers.. (We might
> be better off avoiding an smp_processor_id() if possible, since this function could
> be called in very hot paths).. So I don't see a strong case for returning the
> CPU number. But let me know if you think it'll still be worth it for some reason...

I just noted a lot of two-line code sequences in your patch that would be
one line if the CPU number was returned.  But I don't feel strongly about
it, so if people are OK with the current version, no problem.

							Thanx, Paul

> >  Looks good otherwise.
> > 
> 
> Thank you very much for the detailed review, Paul!
> 
> Regards,
> Srivatsa S. Bhat
> 
> > 
> >> ---
> >>
> >>  kernel/smp.c |   40 ++++++++++++++++++++++++++--------------
> >>  1 file changed, 26 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/kernel/smp.c b/kernel/smp.c
> >> index 29dd40a..f421bcc 100644
> >> --- a/kernel/smp.c
> >> +++ b/kernel/smp.c
> >> @@ -310,7 +310,8 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >>  	 * prevent preemption and reschedule on another processor,
> >>  	 * as well as CPU removal
> >>  	 */
> >> -	this_cpu = get_cpu();
> >> +	get_online_cpus_atomic();
> >> +	this_cpu = smp_processor_id();
> >>
> >>  	/*
> >>  	 * Can deadlock when called with interrupts disabled.
> >> @@ -342,7 +343,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >>  		}
> >>  	}
> >>
> >> -	put_cpu();
> >> +	put_online_cpus_atomic();
> >>
> >>  	return err;
> >>  }
> >> @@ -371,8 +372,10 @@ int smp_call_function_any(const struct cpumask *mask,
> >>  	const struct cpumask *nodemask;
> >>  	int ret;
> >>
> >> +	get_online_cpus_atomic();
> >>  	/* Try for same CPU (cheapest) */
> >> -	cpu = get_cpu();
> >> +	cpu = smp_processor_id();
> >> +
> >>  	if (cpumask_test_cpu(cpu, mask))
> >>  		goto call;
> >>
> >> @@ -388,7 +391,7 @@ int smp_call_function_any(const struct cpumask *mask,
> >>  	cpu = cpumask_any_and(mask, cpu_online_mask);
> >>  call:
> >>  	ret = smp_call_function_single(cpu, func, info, wait);
> >> -	put_cpu();
> >> +	put_online_cpus_atomic();
> >>  	return ret;
> >>  }
> >>  EXPORT_SYMBOL_GPL(smp_call_function_any);
> >> @@ -409,25 +412,28 @@ void __smp_call_function_single(int cpu, struct call_single_data *data,
> >>  	unsigned int this_cpu;
> >>  	unsigned long flags;
> >>
> >> -	this_cpu = get_cpu();
> >> +	get_online_cpus_atomic();
> >> +
> >> +	this_cpu = smp_processor_id();
> >> +
> >>  	/*
> >>  	 * Can deadlock when called with interrupts disabled.
> >>  	 * We allow cpu's that are not yet online though, as no one else can
> >>  	 * send smp call function interrupt to this cpu and as such deadlocks
> >>  	 * can't happen.
> >>  	 */
> >> -	WARN_ON_ONCE(cpu_online(smp_processor_id()) && wait && irqs_disabled()
> >> +	WARN_ON_ONCE(cpu_online(this_cpu) && wait && irqs_disabled()
> >>  		     && !oops_in_progress);
> >>
> >>  	if (cpu == this_cpu) {
> >>  		local_irq_save(flags);
> >>  		data->func(data->info);
> >>  		local_irq_restore(flags);
> >> -	} else {
> >> +	} else if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
> >>  		csd_lock(data);
> >>  		generic_exec_single(cpu, data, wait);
> >>  	}
> >> -	put_cpu();
> >> +	put_online_cpus_atomic();
> >>  }
> >>
> >>  /**
> >> @@ -451,6 +457,8 @@ void smp_call_function_many(const struct cpumask *mask,
> >>  	unsigned long flags;
> >>  	int refs, cpu, next_cpu, this_cpu = smp_processor_id();
> >>
> >> +	get_online_cpus_atomic();
> >> +
> >>  	/*
> >>  	 * Can deadlock when called with interrupts disabled.
> >>  	 * We allow cpu's that are not yet online though, as no one else can
> >> @@ -467,17 +475,18 @@ void smp_call_function_many(const struct cpumask *mask,
> >>
> >>  	/* No online cpus?  We're done. */
> >>  	if (cpu >= nr_cpu_ids)
> >> -		return;
> >> +		goto out_unlock;
> >>
> >>  	/* Do we have another CPU which isn't us? */
> >>  	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> >>  	if (next_cpu == this_cpu)
> >> -		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
> >> +		next_cpu = cpumask_next_and(next_cpu, mask,
> >> +						cpu_online_mask);
> >>
> >>  	/* Fastpath: do that cpu by itself. */
> >>  	if (next_cpu >= nr_cpu_ids) {
> >>  		smp_call_function_single(cpu, func, info, wait);
> >> -		return;
> >> +		goto out_unlock;
> >>  	}
> >>
> >>  	data = &__get_cpu_var(cfd_data);
> >> @@ -523,7 +532,7 @@ void smp_call_function_many(const struct cpumask *mask,
> >>  	/* Some callers race with other cpus changing the passed mask */
> >>  	if (unlikely(!refs)) {
> >>  		csd_unlock(&data->csd);
> >> -		return;
> >> +		goto out_unlock;
> >>  	}
> >>
> >>  	raw_spin_lock_irqsave(&call_function.lock, flags);
> >> @@ -554,6 +563,9 @@ void smp_call_function_many(const struct cpumask *mask,
> >>  	/* Optionally wait for the CPUs to complete */
> >>  	if (wait)
> >>  		csd_lock_wait(&data->csd);
> >> +
> >> +out_unlock:
> >> +	put_online_cpus_atomic();
> >>  }
> >>  EXPORT_SYMBOL(smp_call_function_many);
> >>
> >> @@ -574,9 +586,9 @@ EXPORT_SYMBOL(smp_call_function_many);
> >>   */
> >>  int smp_call_function(smp_call_func_t func, void *info, int wait)
> >>  {
> >> -	preempt_disable();
> >> +	get_online_cpus_atomic();
> >>  	smp_call_function_many(cpu_online_mask, func, info, wait);
> >> -	preempt_enable();
> >> +	put_online_cpus_atomic();
> >>
> >>  	return 0;
> >>  }
> >>
> 

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