Subject: Re: Default enable RCU list lockdep debugging with PROVE_RCU

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 14 May 2020 06:33:28 -0700

On Thu, May 14, 2020 at 08:31:13AM -0400, Qian Cai wrote:
> 
> 
> > On May 14, 2020, at 8:25 AM, Stephen Rothwell <[email-masked]> wrote:
> > 
> > Hi Paul,
> > 
> > This patch in the rcu tree
> > 
> >  d13fee049fa8 ("Default enable RCU list lockdep debugging with PROVE_RCU")
> > 
> > is causing whack-a-mole in the syzbot testing of linux-next.  Because
> > they always do a debug build of linux-next, no testing is getting done. :-(
> > 
> > Can we find another way to find all the bugs that are being discovered
> > (very slowly)?
> 
> Alternatively, could syzbot to use PROVE_RCU=n temporarily because it can’t keep up with it? I personally found PROVE_RCU_LIST=y is still useful for my linux-next testing, and don’t want to lose that coverage overnight.

The problem is that PROVE_RCU is exactly PROVE_LOCKING, and asking people
to test without PROVE_LOCKING is a no-go in my opinion.  But of course
on the other hand if there is no testing of RCU list lockdep debugging,
those issues will never be found, let alone fixed.

One approach would be to do as Stephen asks (either remove d13fee049fa8
or pull it out of -next) and have testers force-enable the RCU list
lockdep debugging.

Would that work for you?

							Thanx, Paul