Re: Default enable RCU list lockdep debugging with PROVE_RCU

Qian Cai
2020-05-14T14:03:27Z

> 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.

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.