Re: [RFC][PATCH 2/5] cpu-hotplug: Allow get_online_cpus() to nest

[Paul E. McKenney]

On Fri, May 12, 2017 at 01:15:46PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> Allow get_online_cpus() to be recursive. If a lock is taken while under
> "get_online_cpus()", it can call get_online_cpus() as well, just as long as
> it is never held without being under get_online_cpus(), but then calling it.
> 
>    GOC() -> Lock(X) -> GOC()
> 
> is OK, as long as
> 
>    Lock(X) -> GOC()
> 
> does not exist.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Does ->goc_depth also need to be initialized in include/linux/init_task.h?

It seems like C-language initialization-to-zero would cover this,
but there is a lot of initialization to zero values in init_task.h
(including but not limited to some RCU stuff).

Other than that, this does look like it might ease use of get_online_cpus()
in combination with CPU-hotplug notifiers, although it would not have
made any difference in any of the RCU use cases.

							Thanx, Paul

> ---
>  include/linux/sched.h | 1 +
>  kernel/cpu.c          | 9 +++++++++
>  kernel/fork.c         | 1 +
>  3 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 993e7e25a3a5..8f272ab57685 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -813,6 +813,7 @@ struct task_struct {
>  	unsigned int			lockdep_recursion;
>  	struct held_lock		held_locks[MAX_LOCK_DEPTH];
>  	gfp_t				lockdep_reclaim_gfp;
> +	int				goc_depth;
>  #endif
> 
>  #ifdef CONFIG_UBSAN
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 983163ef36ee..0dbdf1e69715 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -209,12 +209,21 @@ DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
> 
>  void get_online_cpus(void)
>  {
> +#ifdef CONFIG_LOCKDEP
> +	if (current->goc_depth++)
> +		return;
> +#endif
>  	percpu_down_read(&cpu_hotplug_lock);
>  }
>  EXPORT_SYMBOL_GPL(get_online_cpus);
> 
>  void put_online_cpus(void)
>  {
> +#ifdef CONFIG_LOCKDEP
> +	WARN_ON_ONCE(current->goc_depth < 1);
> +	if (--current->goc_depth)
> +		return;
> +#endif
>  	percpu_up_read(&cpu_hotplug_lock);
>  }
>  EXPORT_SYMBOL_GPL(put_online_cpus);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index dd5a371c392a..8aedcd011ccc 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1658,6 +1658,7 @@ static __latent_entropy struct task_struct *copy_process(
>  	p->lockdep_depth = 0; /* no locks held yet */
>  	p->curr_chain_key = 0;
>  	p->lockdep_recursion = 0;
> +	p->goc_depth = 0;
>  #endif
> 
>  #ifdef CONFIG_DEBUG_MUTEXES
> -- 
> 2.10.2
> 
>