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

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 14 May 2020 08:34:00 -0700

On Thu, May 14, 2020 at 10:03:21AM -0400, Qian Cai wrote:
> 
> 
> > On May 14, 2020, at 9:54 AM, Paul E. McKenney <[email-masked]> wrote:
> > 
> > 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?
> 
> 
> If PROVE_RCU_LIST=n if DEBUG_AID_FOR_SYZBOT works for syzbot guys, that would be great, so other testing agents could still report/fix those RCU-list bugs and then pave a way for syzbot to return back once all those false positives had been sorted out.

On that, I must defer to the syzbot guys.

> Otherwise,  “select PROVE_RCU_LIST” *might* be better than buried into RCU_EXPERT where we will probably never saw those false positives been addressed since my configs does not cover a wide range of subsystems and probably not many other bots would enable RCU_EXPERT.

Yet another option would be to edit your local kernel/rcu/Kconfig.debug
and change the code to the following:

	config PROVE_RCU_LIST
		def_bool y
		help
		  Enable RCU lockdep checking for list usages. It is default
		  enabled with CONFIG_PROVE_RCU.

Removing the RCU_EXPERT dependency would not go over at all well with
some people whose opinions are difficult to ignore.  ;-)

							Thanx, Paul