linux-kernel - Re: [syzbot] [usb?] INFO: task hung in wdm

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Date: Thu, 20 Jun 2024 20:47:03 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+973d01eb49b060b12e63@...kaller.appspotmail.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Boqun Feng <boqun.feng@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [usb?] INFO: task hung in wdm_release

On 2024/06/20 19:38, Hillf Danton wrote:
> On Thu, 20 Jun 2024 02:08:21 -0700
>> Showing all locks held in the system:
>> 3 locks held by kworker/u8:0/11:
>>  #0: ffff8880b953e7d8 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x2a/0x140 kernel/sched/core.c:559
>>  #1: ffff8880b9528948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_switch+0x441/0x770 kernel/sched/psi.c:988
>>  #2: ffff8880754f0768 (&rdev->wiphy.mtx){+.+.}-{3:3}, at: wiphy_lock include/net/cfg80211.h:5966 [inline]
>>  #2: ffff8880754f0768 (&rdev->wiphy.mtx){+.+.}-{3:3}, at: cfg80211_wiphy_work+0x35/0x260 net/wireless/core.c:424
> 
> The info looks bogus given acquiring mutex with runqueue lock held.

Nothing wrong. Printing the backtrace and/or locks held is not atomic.
That is, locks held by a non current thread can change at any moment.

For example, the former block starting with
"INFO: task syz-executor320:7035 blocked for more than 142 seconds."
says that pid 7035 is blocked at "mutex_lock(&wdm_mutex)", but the latter
block starting with "Showing all locks held in the system:" says that
pid 7035 was holding (or trying to hold) no lock.

Threads were making progress slowly. Though, printk() flooding by

  cdc_wdm 5-1:1.0: nonzero urb status received: -71
  cdc_wdm 5-1:1.0: wdm_int_callback - 0 bytes

should be avoided. We need to persuade Greg to let these noisy messages reduced.