Re: [PATCH tip/core/rcu 1/5] rcu: Add duplicate-callback tests to rcutorture

[Josh Triplett <josh@joshtriplett.org>]

On Sun, Aug 18, 2013 at 08:55:28PM -0700, Paul E. McKenney wrote:
> On Sat, Aug 17, 2013 at 07:54:20PM -0700, Josh Triplett wrote:
> > On Sat, Aug 17, 2013 at 07:25:13PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > 
> > > This commit adds a object_debug option to rcutorture to allow the
> > > debug-object-based checks for duplicate call_rcu() invocations to
> > > be deterministically tested.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Sedat Dilek <sedat.dilek@gmail.com>
> > > Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
> > > Cc: Rik van Riel <riel@surriel.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> > 
> > Two comments below; with those fixed,
> > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> > 
> > > ---
> > > @@ -100,6 +101,8 @@ module_param(fqs_stutter, int, 0444);
> > >  MODULE_PARM_DESC(fqs_stutter, "Wait time between fqs bursts (s)");
> > >  module_param(n_barrier_cbs, int, 0444);
> > >  MODULE_PARM_DESC(n_barrier_cbs, "# of callbacks/kthreads for barrier testing");
> > > +module_param(object_debug, int, 0444);
> > > +MODULE_PARM_DESC(object_debug, "Enable debug-object double call_rcu() testing");
> > 
> > modules-next has a change to ignore and warn about
> > unknown module parameters.  Thus, I'd suggest wrapping the ifdef around
> > this module parameter, so it doesn't exist at all without
> > CONFIG_DEBUG_OBJECTS_RCU_HEAD.
> > 
> > Alternatively, consider providing the test unconditionally, and just
> > printing a big warning message saying that it's going to cause
> > corruption in the !CONFIG_DEBUG_OBJECTS_RCU_HEAD case.
> 
> I currently do something like the above.  The module parameter
> is defined unconditionally, but the actual tests are under #ifdef
> CONFIG_DEBUG_OBJECTS_RCU_HEAD.  If you specify object_debug for a
> !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel, the pr_alert() below happens,
> and the test is omitted, thus avoiding the list corruption.
> 
> Seem reasonable?

That's exactly the bit I was commenting on.  I'm saying that you should
either make the test unconditional (perhaps with a warning saying it's
about to cause list corruption), or you should compile out the module
parameter as well and then you don't need the pr_alert (since current
kernels will emit a warning when you pass a non-existent module
parameter).

Personally, I'd go with the latter.

- Josh Triplett