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

[Linus Torvalds]

On Sat, Feb 4, 2023 at 12:01 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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