linux-kernel - Re: Default enable RCU list lockdep debugging with PROVE

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Date:   Thu, 14 May 2020 10:03:21 -0400
From:   Qian Cai <cai@....pw>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        Amol Grover <frextrite@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: Default enable RCU list lockdep debugging with PROVE_RCU



> On May 14, 2020, at 9:54 AM, Paul E. McKenney <paulmck@...nel.org> 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 <paulmck@...nel.org> wrote:
>>> 
>>> On Thu, May 14, 2020 at 08:31:13AM -0400, Qian Cai wrote:
>>>> 
>>>> 
>>>>> On May 14, 2020, at 8:25 AM, Stephen Rothwell <sfr@...b.auug.org.au> 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.

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.