Re: [PATCH -printk] printk, tracing: fix console tracepoint

From: Paul E. McKenney
Date: Tue Jul 12 2022 - 11:21:09 EST

On Tue, Jul 12, 2022 at 10:53:53AM -0400, Steven Rostedt wrote:
> On Tue, 12 Jul 2022 06:49:16 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> > > I guess the question is, can we have printk() in such a place? Because this
> > > tracepoint is attached to printk and where ever printk is done so is this
> > > tracepoint.
> >
> > As I understand it, code in such a place should be labeled noinstr.
> > Then the call to printk() would be complained about as an illegal
> > noinstr-to-non-noinstr call.
> >
> > But where exactly is that printk()?
> Perhaps the fix is to remove the _rcuidle() from trace_console_rcuidle().
> If printk() can never be called from noinstr (aka RCU not watching).

Maybe printk() is supposed to be invoked from noinstr. It might be a
special case in the tooling. I have no idea. ;-)

However, the current SRCU read-side algorithm will tolerate being invoked
from noinstr as long as it is not also an NMI handler. Much though
debugging tools might (or might not) complain.

Don't get me wrong, I can make SRCU tolerate being called while RCU is
not watching. It is not even all that complicated. The cost is that
architectures that have NMIs but do not have NMI-safe this_cpu*()
operations have an SRCU reader switch from explicit smp_mb() and
interrupt disabling to a cmpxchg() loop relying on the implicit barriers
in cmpxchg().

For arm64, this was reportedly a win.

If it turns out that we need SRCU readers to be invoked from NMI handlers
in locations where RCU is not watching, are there people who would be
willing to play with a modified SRCU on the systems in question?

-ENOHARDWARE at this end.

Thanx, Paul