linux-kernel - Re: Converting dev->mutex into dev->spinlock ?

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Message-ID: <e89ad942-cab6-fad1-1cc2-98885d829ea7@I-love.SAKURA.ne.jp>
Date:   Sun, 5 Feb 2023 00:30:07 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: Converting dev->mutex into dev->spinlock ?

On 2023/02/05 0:12, Alan Stern wrote:
>>  it would solve many deadlocks in driver code if you can update
> 
> What deadlocks?  If there are so many deadlocks floating around in 
> driver code, why haven't we heard about them before now?

Since dev->mutex is hidden from lockdep checks, nobody can see lockdep warnings.
syzbot is reporting real deadlocks without lockdep warnings, for the fundamental
problem you mentioned in https://lkml.kernel.org/r/Pine.LNX.4.44L0.0804171117450.18040-100000@iolanthe.rowland.org
is remaining. I'm suggesting you that now is time to address this fundamental problem.

>> (by e.g. replacing dev->mutex with dev->spinlock and dev->atomic_flags).
>> But I'm not familiar enough to propose such change...
> 
> Such a change cannot be made.  Consider this: Driver callbacks often
> need to sleep.  But when a thread holds a spinlock, it is not allowed to 
> sleep.  Therefore driver callbacks must not be invoked while a spinlock 
> is held.

What I'm suggesting is "Do not call driver callbacks with dev->mutex held,
by rewriting driver core code".