linux-kernel - Re: [syzbot] [kernel?] possible deadlock in console_flush

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Date: Tue, 19 Mar 2024 23:56:31 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: syzbot <syzbot+f78380e4eae53c64125c@...kaller.appspotmail.com>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
	John Ogness <john.ogness@...utronix.de>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [syzbot] [kernel?] possible deadlock in console_flush_all (2)

On Tue, Mar 19, 2024 at 11:50 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Hello, bpf developers.
>
> syzbot is reporting circular locking dependency because bpf program is hitting
> printk() path. To avoid this dependency, something like
> https://syzkaller.appspot.com/text?tag=Patch&x=121c92fe180000 is needed.
>
> Is it possible to call printk_deferred_{enter,exit}() from the bpf side (e.g.
> bpf_prog_run()) so that we don't need to add overhead on the scheduler side
> when bpf programs are not registered?
>
> printk_deferred_{enter,exit}() requires that printk_deferred_exit() is called
> on a CPU which printk_deferred_enter() was called, for percpu counter is used.
> Is migrate_{disable,enable}() also needed around printk_deferred_{enter,exit}() ?
>
> On 2023/10/08 15:28, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    7d730f1bf6f3 Add linux-next specific files for 20231005
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=172d36e9680000

This has nothing to do with bpf.
bpf never calls printk().