linux-kernel - Re: [PATCH] char: misc: make misc_open() and misc

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Date:   Mon, 4 Jul 2022 13:01:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     arnd@...db.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] char: misc: make misc_open() and misc_register() killable

On Mon, Jul 04, 2022 at 07:25:44PM +0900, Tetsuo Handa wrote:
> On 2022/07/04 16:29, Greg KH wrote:
> > On Mon, Jul 04, 2022 at 03:44:07PM +0900, Tetsuo Handa wrote:
> >> syzbot is reporting hung task at misc_open() [1], for snapshot_open() from
> >> misc_open() might sleep for long with misc_mtx held whereas userspace can
> >> flood with concurrent misc_open() requests. Mitigate this problem by making
> >> misc_open() and misc_register() killable.
> > 
> > I do not understand, why not just fix snapshot_open()?  Why add this
> > complexity to the misc core for a foolish individual misc device?  Why
> > not add the fix there where it is spinning instead?
> 
> Quoting an example from [1]. Multiple processes are calling misc_open() and
> all but one processes are blocked at mutex_lock(&misc_mtx). The one which is
> not blocked at mutex_lock(&misc_mtx) is also holding system_transition_mutex.

And that is because of that one misc device, right?  Why not fix that
instead of papering over the issue in the misc core?

thanks,

greg k-h