linux-kernel - Re: [syzbot] INFO: task hung in rfkill_global_led_trigger

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Message-ID: <8a3964bd-41fa-5179-1916-565decd11f50@I-love.SAKURA.ne.jp>
Date:   Wed, 16 Nov 2022 18:27:45 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+2e39bc6569d281acbcfb@...kaller.appspotmail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] INFO: task hung in rfkill_global_led_trigger_worker (2)

On 2022/11/16 13:17, Hillf Danton wrote:
> No deadlock was reported for syz-executor145/4505 and syz-executor145/4516.
> Why?

Lockdep should be able to report this real deadlock case. But unfortunately,

>> INFO: task syz-executor145:4505 blocked for more than 143 seconds.
>>       Not tainted 6.1.0-rc5-syzkaller-00008-ge01d50cbd6ee #0
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:syz-executor145 state:D stack:21896 pid:4505  ppid:3645   flags:0x00004002
>> 
>> 2 locks held by syz-executor145/4505:
>>  #0: ffff88807268e100 (&dev->mutex){....}-{3:3}, at: device_lock include/linux/device.h:835 [inline]
>>  #0: ffff88807268e100 (&dev->mutex){....}-{3:3}, at: nfc_unregister_device+0x87/0x290 net/nfc/core.c:1165
>>  #1: ffffffff8e787b08 (rfkill_global_mutex){+.+.}-{3:3}, at: rfkill_unregister+0xcb/0x220 net/rfkill/core.c:1130
>> 
>> INFO: task syz-executor145:4516 blocked for more than 144 seconds.
>>       Not tainted 6.1.0-rc5-syzkaller-00008-ge01d50cbd6ee #0
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:syz-executor145 state:D stack:23096 pid:4516  ppid:3647   flags:0x00004004
>> 
>> 2 locks held by syz-executor145/4516:
>>  #0: ffffffff8e787b08 (rfkill_global_mutex){+.+.}-{3:3}, at: rfkill_fop_write+0x1b3/0x790 net/rfkill/core.c:1278
>>  #1: ffff88807268e100 (&dev->mutex){....}-{3:3}, at: device_lock include/linux/device.h:835 [inline]
>>  #1: ffff88807268e100 (&dev->mutex){....}-{3:3}, at: nfc_dev_down+0x33/0x260 net/nfc/core.c:143

device_initialize() hides dev->mutex from lockdep tests; explained at
https://groups.google.com/g/syzkaller-bugs/c/Uj9LqEUCwac/m/BhdTjWhNAQAJ .