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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 4 Feb 2023 12:14:54 -0800
Message-ID: <CAHk-=wjoy=hObTmyRb9ttApjndt0LfqAfv71Cz+hEGrT0cLN+A@mail.gmail.com>
Subject: Re: Converting dev->mutex into dev->spinlock ?
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        Hillf Danton <hdanton@sina.com>
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On Sat, Feb 4, 2023 at 12:01 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> I'm sorry, but that simply is not feasible.  It doesn't matter how much
> you want to do it or feel it is needed; there is no reasonable way to do
> it.  To take just one example, what you are saying implies that when a
> driver is probed for a device, it would not be allowed to register a
> child device.  That's a ridiculous restriction.

Well, we've worked around that in other places by making the lockdep
classes for different locks of the same type be different.

So this *could* possibly be solved by lockdep being smarter about
dev->mutex than just "disable checking entirely".

So maybe the lock_set_novalidate_class() could be something better. It
_is_ kind of disgusting.

That said, maybe people tried to subclass the locks and failed, and
that "no validation" is the best that can be done.

But other areas *do* end up spending extra effort to separate out the
locks (and the different uses of the locks), and I think the
dev->mutex is one of the few cases that just gives up and says "no
validation at all".

The other case seems to be the md bcache code.

                  Linus