[PATCH v2 3/4] jump_label: Provide hotplug context variants

[marc.zyngier@arm.com (Marc Zyngier)]

On 02/08/17 15:37, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 09:02:56AM +0100, Marc Zyngier wrote:
>> As using the normal static key API under the hotplug lock is
>> pretty much impossible, let's provide a variant of some of them
>> that require the hotplug lock to have already been taken.
>>
>> These function are only meant to be used in CPU hotplug callbacks.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  Documentation/static-keys.txt | 15 +++++++++++++++
>>  include/linux/jump_label.h    | 11 +++++++++--
>>  kernel/jump_label.c           | 20 ++++++++++++++++++++
>>  3 files changed, 44 insertions(+), 2 deletions(-)
> 
> I stuffed them in my locking/core branch on top of Paolo's patches:
> 
>   https://lkml.kernel.org/r/1501601046-35683-1-git-send-email-pbonzini at redhat.com
> 
> This patch now looks like so.
> 
> ---
> Subject: jump_label: Provide hotplug context variants
> From:   Marc Zyngier <marc.zyngier@arm.com>
> Date: Tue, 1 Aug 2017 09:02:56 +0100
> 
> As using the normal static key API under the hotplug lock is
> pretty much impossible, let's provide a variant of some of them
> that require the hotplug lock to have already been taken.
> 
> These function are only meant to be used in CPU hotplug callbacks.
> 
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: Leo Yan <leo.yan@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: http://lkml.kernel.org/r/20170801080257.5056-4-marc.zyngier at arm.com
> ---
>  Documentation/static-keys.txt |   15 +++++++++++++++
>  include/linux/jump_label.h    |   11 +++++++++--
>  kernel/jump_label.c           |   22 ++++++++++++++++++----
>  3 files changed, 42 insertions(+), 6 deletions(-)
> 
> --- a/Documentation/static-keys.txt
> +++ b/Documentation/static-keys.txt
> @@ -154,6 +154,21 @@ and 'static_key_count()'.  In general, i
>  should be protected with the same mutex used around the enable/disable
>  or increment/decrement function.
>  
> +Note that switching branches results in some locks being taken,
> +particularly the CPU hotplug lock (in order to avoid races against
> +CPUs being brought in the kernel whilst the kernel is getting
> +patched). Calling the static key API from within a hotplug notifier is
> +thus a sure deadlock recipe. In order to still allow use of the
> +functionnality, the following functions are provided:
> +
> +	static_key_enable_cpuslocked()
> +	static_key_disable_cpuslocked()
> +	static_branch_enable_cpuslocked()
> +	static_branch_disable_cpuslocked()
> +
> +These functions are *not* general purpose, and must only be used when
> +you really know that you're in the above context, and no other.
> +
>  Where an array of keys is required, it can be defined as::
>  
>  	DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -163,6 +163,8 @@ extern void jump_label_apply_nops(struct
>  extern int static_key_count(struct static_key *key);
>  extern void static_key_enable(struct static_key *key);
>  extern void static_key_disable(struct static_key *key);
> +extern void static_key_enable_cpuslocked(struct static_key *key);
> +extern void static_key_disable_cpuslocked(struct static_key *key);
>  
>  /*
>   * We should be using ATOMIC_INIT() for initializing .enabled, but
> @@ -254,6 +256,9 @@ static inline void static_key_disable(st
>  	atomic_set(&key->enabled, 0);
>  }
>  
> +#define static_key_enable_cpuslocked(k)		static_key_enable((k))
> +#define static_key_disable_cpuslocked(k)	static_key_disable((k))
> +
>  #define STATIC_KEY_INIT_TRUE	{ .enabled = ATOMIC_INIT(1) }
>  #define STATIC_KEY_INIT_FALSE	{ .enabled = ATOMIC_INIT(0) }
>  
> @@ -415,8 +420,10 @@ extern bool ____wrong_branch_error(void)
>   * Normal usage; boolean enable/disable.
>   */
>  
> -#define static_branch_enable(x)		static_key_enable(&(x)->key)
> -#define static_branch_disable(x)	static_key_disable(&(x)->key)
> +#define static_branch_enable(x)			static_key_enable(&(x)->key)
> +#define static_branch_disable(x)		static_key_disable(&(x)->key)
> +#define static_branch_enable_cpuslocked(x)	static_key_enable_cpuslocked(&(x)->key)
> +#define static_branch_disable_cpuslocked(x)	static_key_disable_cpuslocked(&(x)->key)
>  
>  #endif /* __ASSEMBLY__ */
>  
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -126,15 +126,15 @@ void static_key_slow_inc(struct static_k
>  }
>  EXPORT_SYMBOL_GPL(static_key_slow_inc);
>  
> -void static_key_enable(struct static_key *key)
> +void static_key_enable_cpuslocked(struct static_key *key)
>  {
>  	STATIC_KEY_CHECK_USE();
> +
>  	if (atomic_read(&key->enabled) > 0) {
>  		WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
>  		return;
>  	}
>  
> -	cpus_read_lock();
>  	jump_label_lock();
>  	if (atomic_read(&key->enabled) == 0) {
>  		atomic_set(&key->enabled, -1);
> @@ -145,23 +145,37 @@ void static_key_enable(struct static_key
>  		atomic_set_release(&key->enabled, 1);
>  	}
>  	jump_label_unlock();
> +}
> +EXPORT_SYMBOL_GPL(static_key_enable_cpuslocked);
> +
> +void static_key_enable(struct static_key *key)
> +{
> +	cpus_read_lock();
> +	static_key_enable_cpuslocked(key);
>  	cpus_read_unlock();
>  }
>  EXPORT_SYMBOL_GPL(static_key_enable);
>  
> -void static_key_disable(struct static_key *key)
> +void static_key_disable_cpuslocked(struct static_key *key)
>  {
>  	STATIC_KEY_CHECK_USE();
> +
>  	if (atomic_read(&key->enabled) != 1) {
>  		WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
>  		return;
>  	}
>  
> -	cpus_read_lock();
>  	jump_label_lock();
>  	if (atomic_cmpxchg(&key->enabled, 1, 0))
>  		jump_label_update(key);
>  	jump_label_unlock();
> +}
> +EXPORT_SYMBOL_GPL(static_key_disable_cpuslocked);
> +
> +void static_key_disable(struct static_key *key)
> +{
> +	cpus_read_lock();
> +	static_key_disable_cpuslocked(key);
>  	cpus_read_unlock();
>  }
>  EXPORT_SYMBOL_GPL(static_key_disable);
> 

I've just tried this one of the affected systems, and it booted just
fine. So thumbs up from me.

Thanks again,

	M.
-- 
Jazz is not dead. It just smells funny...