linux-kernel - Re: [RFC][PATCH 2/5] cpu-hotplug: Allow get_online

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Message-Id: <20170512185218.GD3956@linux.vnet.ibm.com>
Date:   Fri, 12 May 2017 11:52:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 2/5] cpu-hotplug: Allow get_online_cpus() to nest

On Fri, May 12, 2017 at 02:40:27PM -0400, Steven Rostedt wrote:
> On Fri, 12 May 2017 11:35:59 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Fri, May 12, 2017 at 01:15:46PM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > > 
> > > 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@...dmis.org>  
> > 
> > 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).
> 
> I assumed that it would just initialize it to zero.
> 
> OK, I need to add this:
> 
> -- Steve
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index fffe49f..be7f71b 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -375,7 +375,7 @@ extern struct pin_cookie lock_pin_lock(struct lockdep_map *lock);
>  extern void lock_repin_lock(struct lockdep_map *lock, struct pin_cookie);
>  extern void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie);
> 
> -# define INIT_LOCKDEP				.lockdep_recursion = 0, .lockdep_reclaim_gfp = 0,
> +# define INIT_LOCKDEP				.lockdep_recursion = 0, .lockdep_reclaim_gfp = 0, .goc_depth = 0,
> 
>  #define lockdep_depth(tsk)	(debug_locks ? (tsk)->lockdep_depth : 0)

Or maybe we should remove a bunch of zero-initialization from that file.
But if it is needed, then that addition to the patch looks good to me.
Given how much zero-initialization there is, I suspect that it is needed
for some strange boot-up reason.  Hard to believe that someone would not
have gotten rid of it otherwise.

							Thanx, Paul