linux-kernel - Re: INFO: rcu detected stall in sys

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Message-ID: <20190313163701.GE672@mit.edu>
Date:   Wed, 13 Mar 2019 12:37:01 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzbot <syzbot+1505c80c74256c6118a5@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: INFO: rcu detected stall in sys_sendfile64 (2)

On Wed, Mar 13, 2019 at 07:43:38AM +0100, Dmitry Vyukov wrote:
> It would be more useful to accept patches that make syzkaller create
> better reproducers from these people. Manual work is not scalable. We
> would need 10 reproducers per day for a dozen of OSes (incl some
> private kernels/branches). Anybody is free to run syzkaller manually
> and do full manual (perfect) reporting. But for us it become clear
> very early that it won't work. Then see above, while that human is
> sleeping/on weekend/vacation, syzbot will already bisect own
> reproducer. Adding manual reproducer later won't help in any way.
> syzkaller already does lots of smart work for reproducers. Let's not
> give up on the last mile and switch back to all manual work.

I suspect a scalable solution that would significantly improve things
is one where Syzbot tries N times for a "good" result to make sure
it's not a flaky pass.  N could either be hard-coded to some value
like 8 or 10, or Syzbot could experimentally try to figure out how
reliable the reproducer happens to be, and figure out what an ideal
"N" value should be for a particular reproducer.

					- Ted