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Date: Thu, 20 Jun 2024 14:31:17 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Hillf Danton <hdanton@sina.com>,
	syzbot <syzbot+973d01eb49b060b12e63@syzkaller.appspotmail.com>,
	linux-kernel@vger.kernel.org, Boqun Feng <boqun.feng@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-usb@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [usb?] INFO: task hung in wdm_release
Message-ID: <2024062045-improve-tidbit-ed0d@gregkh>
References: <20240620103847.1724-1-hdanton@sina.com>
 <0044d7b1-ae7d-4d36-b730-38b06186c8bf@I-love.SAKURA.ne.jp>
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In-Reply-To: <0044d7b1-ae7d-4d36-b730-38b06186c8bf@I-love.SAKURA.ne.jp>

On Thu, Jun 20, 2024 at 08:47:03PM +0900, Tetsuo Handa wrote:
> 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.

Please see:
	22f008128625 ("USB: class: cdc-wdm: Fix CPU lockup caused by excessive log messages")
it should have fixed this already.