linux-kernel - [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic

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Date:   Sat, 16 Mar 2019 11:09:22 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic
 - not syncing: panic_on_warn set ..."

Hello.

I found a corrupted report at https://syzkaller.appspot.com/text?tag=CrashLog&x=17c6b82b200000 .

The panic() was caused by by WARN_ON() from generic_make_request_checks(), but there
was no printk() messages from WARN_ON(). Moreover, there was no printk() messages
for (at least) nearly one minute despite syzbot's testing likely generates constant
printk() messages. Unfortunately, since the report was truncated due to size limit,
we can't check when was the last time prink() succeeded writing messages to consoles.

----------
14:27:02 executing program 1:
(...snipped...)
14:28:00 executing program 3:
(...snipped...)
[  974.068065][T22281] Kernel panic - not syncing: panic_on_warn set ...
[  974.085454][T22281] CPU: 1 PID: 22281 Comm: syz-executor.1 Not tainted 5.0.0+ #23
[  974.093121][T22281] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
[  974.103193][T22281] Call Trace:
[  974.106513][T22281]  dump_stack+0x172/0x1f0
[  974.110893][T22281]  ? generic_make_request_checks+0x1be0/0x2160
[  974.117235][T22281]  panic+0x2cb/0x65c
[  974.121434][T22281]  ? __warn_printk+0xf3/0xf3
[  974.126064][T22281]  ? generic_make_request_checks+0x1ca3/0x2160
[  974.132257][T22281]  ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
[  974.138521][T22281]  ? __warn.cold+0x5/0x45
[  974.142877][T22281]  ? __warn+0xe8/0x1d0
[  974.146978][T22281]  ? generic_make_request_checks+0x1ca3/0x2160
[  974.153171][T22281]  __warn.cold+0x20/0x45
[  974.157483][T22281]  ? vprintk_emit+0x1ce/0x6d0
[  974.162202][T22281]  ? generic_make_request_checks+0x1ca3/0x2160
[  974.168403][T22281]  report_bug+0x263/0x2b0
[  974.172847][T22281]  do_error_trap+0x11b/0x200
[  974.177646][T22281]  do_invalid_op+0x37/0x50
[  974.182329][T22281]  ? generic_make_request_checks+0x1ca3/0x2160
[  974.188951][T22281]  invalid_op+0x14/0x20
[  974.193142][T22281] RIP: 0010:generic_make_request_checks+0x1ca3/0x2160
----------

Is it possible that the reason there was no printk() messages is that
someone kept printing messages to console from userspace such that
console_trylock() from printk() was not able to succeed for (at least)
nearly one minute?