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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com>

Would it make sense for get_online_cpus_atomic() to return the current
CPU number?  Looks good otherwise.

							Thanx, Paul

> ---
> 
>  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;
>  }
>