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

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Message-ID: <491ff1c3-91d6-eaa7-f551-46a4f8b90f5a@i-love.sakura.ne.jp>
Date:   Tue, 12 Mar 2019 23:29:41 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     syzbot <syzbot+1505c80c74256c6118a5@...kaller.appspotmail.com>,
        dvyukov@...gle.com, syzkaller-bugs@...glegroups.com
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: INFO: rcu detected stall in sys_sendfile64 (2)

(Moving most recipients to bcc: in order to avoid flooding.)

On 2019/03/12 13:08, Al Viro wrote:
> Umm...  Might be a good idea to add some plausibility filters - it is,
> in theory, possible that adding a line in a comment changes behaviour
> (without compiler bugs, even - playing with __LINE__ is all it would
> take), but the odds that it's _not_ a false positive are very low.

Well, 108 out of 168 tests done during this bisection failed to test.
With such high failure ratio, it is possible that by chance no crash
happened during few tests for specific commit; causing a wrong bisection
result. I expect that when trying to conclude "git bisect good" for
specific commit, the tests should be repeated until no crash happened
during 8 successful tests.

Also, this bisection is finding multiple different crash patterns, which
suggests that the crashed tests are not giving correct feedback to syzbot.

$ grep -F 'run #' bisect.txt\?x\=13220283200000 | wc -l
168
$ grep -F 'Connection timed out' bisect.txt\?x\=13220283200000 | wc -l
108
$ grep -F 'crashed' bisect.txt\?x\=13220283200000
run #0: crashed: WARNING: ODEBUG bug in netdev_freemem
run #0: crashed: WARNING: ODEBUG bug in netdev_freemem
run #1: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in sys_sendfile64
run #0: crashed: INFO: rcu detected stall in corrupted
run #4: crashed: INFO: rcu detected stall in sys_sendfile64
run #0: crashed: INFO: rcu detected stall in corrupted
run #1: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in ext4_file_write_iter
run #0: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in sendfile64
run #0: crashed: INFO: rcu detected stall in corrupted
run #1: crashed: INFO: rcu detected stall in sendfile64
run #0: crashed: INFO: rcu detected stall in ext4_file_write_iter
run #1: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in corrupted
run #1: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in corrupted
run #3: crashed: INFO: rcu detected stall in corrupted
run #0: crashed: INFO: rcu detected stall in do_iter_write