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

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Message-ID: <ZhAbqvXeeZ0mz2ZX@slm.duckdns.org>
Date: Fri, 5 Apr 2024 05:41:30 -1000
From: Tejun Heo <tj@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Amir Goldstein <amir73il@...il.com>, Al Viro <viro@...iv.linux.org.uk>,
	syzbot <syzbot+9a5b0ced8b1bfb238b56@...kaller.appspotmail.com>,
	gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
	valesini@...dex-team.ru, Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>, Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek

Hello,

On Fri, Apr 05, 2024 at 08:51:35AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 04, 2024 at 12:33:40PM +0300, Amir Goldstein wrote:
> > I don't follow what you are saying.
> > Which code is in non-starter violation?
> > kernfs for calling lookup_bdev() with internal of->mutex held?
> 
> That is a huge problem, and has been causing endless annoying lockdep
> chains in the block layer for us.  If we have some way to kill this
> the whole block layer would benefit.

of->mutex is mostly there as a convenience to kernfs (here, sysfs) users so
that they don't have to worry about concurrent invocation of the callbacks.
It needs more careful look but on cursory observation, it shouldn't be
difficult to implement a flag or different op type which skips of->mutex if
this causes a lot of pain.

Thanks.

-- 
tejun