Subject: Re: [PATCH v4 4/4] pseries: Serialize cpu hotplug operations during deactivate Vs deallocate

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 27 Oct 2009 14:23:10 +1100

On Fri, 2009-10-09 at 14:01 +0530, Gautham R Shenoy wrote:
> Currently the cpu-allocation/deallocation process comprises of two steps:
> - Set the indicators and to update the device tree with DLPAR node
>   information.
> 
> - Online/offline the allocated/deallocated CPU.
> 
> This is achieved by writing to the sysfs tunables "probe" during allocation
> and "release" during deallocation.
> 
> At the sametime, the userspace can independently online/offline the CPUs of
> the system using the sysfs tunable "online".
> 
> It is quite possible that when a userspace tool offlines a CPU
> for the purpose of deallocation and is in the process of updating the device
> tree, some other userspace tool could bring the CPU back online by writing to
> the "online" sysfs tunable thereby causing the deallocate process to fail.
> 
> The solution to this is to serialize writes to the "probe/release" sysfs
> tunable with the writes to the "online" sysfs tunable.
> 
> This patch employs a mutex to provide this serialization, which is a no-op on
> all architectures except PPC_PSERIES
> 
> Signed-off-by: Gautham R Shenoy <[email-masked]>

Peter, did you get a chance to review this one ?

Cheers,
Ben.

> ---
>  arch/powerpc/platforms/pseries/dlpar.c |   26 ++++++++++++++++++++++----
>  drivers/base/cpu.c                     |    2 ++
>  include/linux/cpu.h                    |   13 +++++++++++++
>  3 files changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/dlpar.c b/arch/powerpc/platforms/pseries/dlpar.c
> index 9752386..fc261e6 100644
> --- a/arch/powerpc/platforms/pseries/dlpar.c
> +++ b/arch/powerpc/platforms/pseries/dlpar.c
> @@ -644,6 +644,18 @@ static ssize_t memory_release_store(struct class *class, const char *buf,
>  	return rc ? -1 : count;
>  }
>  
> +static DEFINE_MUTEX(pseries_cpu_hotplug_mutex);
> +
> +void cpu_hotplug_driver_lock()
> +{
> +	mutex_lock(&pseries_cpu_hotplug_mutex);
> +}
> +
> +void cpu_hotplug_driver_unlock()
> +{
> +	mutex_unlock(&pseries_cpu_hotplug_mutex);
> +}
> +
>  static ssize_t cpu_probe_store(struct class *class, const char *buf,
>  			       size_t count)
>  {
> @@ -656,14 +668,15 @@ static ssize_t cpu_probe_store(struct class *class, const char *buf,
>  	if (rc)
>  		return -EINVAL;
>  
> +	cpu_hotplug_driver_lock();
>  	rc = acquire_drc(drc_index);
>  	if (rc)
> -		return rc;
> +		goto out;
>  
>  	dn = configure_connector(drc_index);
>  	if (!dn) {
>  		release_drc(drc_index);
> -		return rc;
> +		goto out;
>  	}
>  
>  	/* fixup dn name */
> @@ -672,7 +685,8 @@ static ssize_t cpu_probe_store(struct class *class, const char *buf,
>  	if (!cpu_name) {
>  		free_cc_nodes(dn);
>  		release_drc(drc_index);
> -		return -ENOMEM;
> +		rc = -ENOMEM;
> +		goto out;
>  	}
>  
>  	sprintf(cpu_name, "/cpus/%s", dn->full_name);
> @@ -684,6 +698,8 @@ static ssize_t cpu_probe_store(struct class *class, const char *buf,
>  		release_drc(drc_index);
>  
>  	rc = online_node_cpus(dn);
> +out:
> +	cpu_hotplug_driver_unlock();
>  
>  	return rc ? rc : count;
>  }
> @@ -705,6 +721,7 @@ static ssize_t cpu_release_store(struct class *class, const char *buf,
>  		return -EINVAL;
>  	}
>  
> +	cpu_hotplug_driver_lock();
>  	rc = offline_node_cpus(dn);
>  
>  	if (rc)
> @@ -713,7 +730,7 @@ static ssize_t cpu_release_store(struct class *class, const char *buf,
>  	rc = release_drc(*drc_index);
>  	if (rc) {
>  		of_node_put(dn);
> -		return rc;
> +		goto out;
>  	}
>  
>  	rc = remove_device_tree_nodes(dn);
> @@ -723,6 +740,7 @@ static ssize_t cpu_release_store(struct class *class, const char *buf,
>  	of_node_put(dn);
>  
>  out:
> +	cpu_hotplug_driver_unlock();
>  	return rc ? rc : count;
>  }
>  
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index e62a4cc..07c3f05 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -35,6 +35,7 @@ static ssize_t __ref store_online(struct sys_device *dev, struct sysdev_attribut
>  	struct cpu *cpu = container_of(dev, struct cpu, sysdev);
>  	ssize_t ret;
>  
> +	cpu_hotplug_driver_lock();
>  	switch (buf[0]) {
>  	case '0':
>  		ret = cpu_down(cpu->sysdev.id);
> @@ -49,6 +50,7 @@ static ssize_t __ref store_online(struct sys_device *dev, struct sysdev_attribut
>  	default:
>  		ret = -EINVAL;
>  	}
> +	cpu_hotplug_driver_unlock();
>  
>  	if (ret >= 0)
>  		ret = count;
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 4753619..b0ad4e1 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -115,6 +115,19 @@ extern void put_online_cpus(void);
>  #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
>  int cpu_down(unsigned int cpu);
>  
> +#ifdef CONFIG_PPC_PSERIES
> +extern void cpu_hotplug_driver_lock(void);
> +extern void cpu_hotplug_driver_unlock(void);
> +#else
> +static inline void cpu_hotplug_driver_lock(void)
> +{
> +}
> +
> +static inline void cpu_hotplug_driver_unlock(void)
> +{
> +}
> +#endif
> +
>  #else		/* CONFIG_HOTPLUG_CPU */
>  
>  #define get_online_cpus()	do { } while (0)
> 
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