Re: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early_initcall()

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 14 Dec 2014 10:08:54 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Sun, Dec 14, 2014 at 11:53:32AM -0500, Steven Rostedt wrote:
> > On Sun, 14 Dec 2014 11:41:05 -0500
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> > > 
> > > In order to move enabling of trace events to just after mm_init(), the
> > > tracepoint enable code can not use call_rcu_sched() because rcu isn't
> > > even initialized yet.  Since this can only happen before SMP is set up
> > > (and even before interrupts are set up), there's no reason to use
> > > call_rcu_sched() at this point.
> > > 
> > > Instead, create a variable called tracepoint_rcu_safe that gets enabled
> > > via early_initcall() and if that is not set, free the code directly
> > > instead of using call_rcu_sched().
> > > 
> > > This allows us to enable tracepoints early without issues.
> > > 
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> 
> With the addition of read_mostly, and given that I am not going to mess
> with call_rcu() this late in the 3.19 process without a blazingly good
> reason:
> 
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Thanks!

> 
> Please note that you can use call_rcu() and friends as soon as rcu_init()
> returns.  The callbacks won't be invoked until early_initcall() time,
> but they will be properly queued.
> 
> Please note also that there are places where turning a call_rcu() into
> a direct function call don't work, even at times when preemption is
> disabled and there is only one CPU.  One example is where single-threaded
> code uses call_rcu() on a list element of a list that it is traversing
> within an RCU read-side critical section.  A direct call to the RCU
> callback could potentially destroy the pointers that the traversal was
> going to use to find the next element.  This means that we cannot make
> call_rcu() do direct calls to the callback, as that would break quite
> a bit of existing code.
> 
> Is there some definite point during boot before which you won't need to
> invoke call_rcu_sched() for tracing?  I am guessing "no", but have to ask.
> I can probably make call_rcu_sched() work arbitrarily early, but it is a
> bit uglier.  And this assumes that irqs_disabled_flags(local_irq_save())
> returns true during early boot.  I would -hope- this would be true!  ;-)
> 

With your feed back, and because I would like this to go into 3.19, I
would like to keep the current patch as is (with the read_mostly
update, which I'm currently testing). We can always change it later
after call_rcu() has been changed.

-- Steve