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

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 14 May 2020 06:54:02 -0700

On Thu, May 14, 2020 at 09:44:28AM -0400, Qian Cai wrote:
> 
> 
> > On May 14, 2020, at 9:33 AM, Paul E. McKenney <[email-masked]> wrote:
> > 
> > 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?
> 
> Alternatively, how about having
> 
> PROVE_RCU_LIST=n if DEBUG_AID_FOR_SYZBOT
> 
> since it is only syzbot can’t keep up with it?

Sound good to me, assuming that this works for the syzkaller guys.
Or could there be a "select PROVE_RCU_LIST" for the people who would
like to test it.

Alternatively, if we revert d13fee049fa8 from -next, I could provide
you a script that updates your .config to set both RCU_EXPERT and
PROVE_RCU_LIST.

There are a lot of ways to appraoch this.

So what would work best for everyone?

							Thanx, Paul