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

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Date:   Thu, 14 May 2020 11:46:23 -0400
From:   Qian Cai <cai@....pw>
To:     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 11:34 AM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Thu, May 14, 2020 at 10:03:21AM -0400, Qian Cai wrote:
>> 
>> 
>>> 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.
> 
> 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.  ;-)

I am trying to not getting into a game of carrying any custom patch myself.

Let’s see what syzbot guys will say, and then I’ll enable RCU_EXPERT myself if needed, but again we probably never see PROVE_RCU_LIST to be used again in syzbot for this path. I surely have no cycles to expand the testing coverage for more subsystems at the moment.