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Subject: Re: Default enable RCU list lockdep debugging with PROVE_RCU
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200514135402.GI2869@paulmck-ThinkPad-P72>
Date:   Thu, 14 May 2020 10:03:21 -0400
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        Amol Grover <frextrite@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>
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References: <20200514222535.259cb69e@canb.auug.org.au>
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 <20200514133328.GG2869@paulmck-ThinkPad-P72>
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To:     "Paul E. McKenney" <paulmck@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 14, 2020, at 9:54 AM, Paul E. McKenney <paulmck@kernel.org> =
wrote:
>=20
> On Thu, May 14, 2020 at 09:44:28AM -0400, Qian Cai wrote:
>>=20
>>=20
>>> On May 14, 2020, at 9:33 AM, Paul E. McKenney <paulmck@kernel.org> =
wrote:
>>>=20
>>> On Thu, May 14, 2020 at 08:31:13AM -0400, Qian Cai wrote:
>>>>=20
>>>>=20
>>>>> On May 14, 2020, at 8:25 AM, Stephen Rothwell =
<sfr@canb.auug.org.au> wrote:
>>>>>=20
>>>>> Hi Paul,
>>>>>=20
>>>>> This patch in the rcu tree
>>>>>=20
>>>>> d13fee049fa8 ("Default enable RCU list lockdep debugging with =
PROVE_RCU")
>>>>>=20
>>>>> 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. :-(
>>>>>=20
>>>>> Can we find another way to find all the bugs that are being =
discovered
>>>>> (very slowly)?
>>>>=20
>>>> Alternatively, could syzbot to use PROVE_RCU=3Dn temporarily =
because it can=E2=80=99t keep up with it? I personally found =
PROVE_RCU_LIST=3Dy is still useful for my linux-next testing, and =
don=E2=80=99t want to lose that coverage overnight.
>>>=20
>>> 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.
>>>=20
>>> 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.
>>>=20
>>> Would that work for you?
>>=20
>> Alternatively, how about having
>>=20
>> PROVE_RCU_LIST=3Dn if DEBUG_AID_FOR_SYZBOT
>>=20
>> since it is only syzbot can=E2=80=99t keep up with it?
>=20
> 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.
>=20
> 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.
>=20
> There are a lot of ways to appraoch this.
>=20
> So what would work best for everyone?


If PROVE_RCU_LIST=3Dn 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,  =E2=80=9Cselect PROVE_RCU_LIST=E2=80=9D *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.=