Re: Default enable RCU list lockdep debugging with PROVE_RCU

Qian Cai
2020-05-14T15:46:31Z

> On May 14, 2020, at 11:34 AM, Paul E. McKenney <paulmck@kernel.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@kernel.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@kernel.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@canb.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.