linux-kernel - Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate

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Date:   Mon, 13 Feb 2023 18:51:57 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzkaller <syzkaller@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate_class()
 with unique class keys

On Mon, Feb 13, 2023 at 11:18:50AM -0500, Alan Stern wrote:
> On Mon, Feb 13, 2023 at 10:49:50AM +0100, Peter Zijlstra wrote:
> > My main worry when adding a ton of classes like this is the
> > combinatorics (dynamic classes make this more trivial, but it's entirely
> > possible with just static classes too).
> > 
> > As an example, we used to have a static class per cpu runqueue, now,
> > given migration takes two runqueue locks (per locking rules in cpu
> > order -- source and dest runqueue etc..) we got O(n^2) combinations of
> > class orderings, which lead to a giant graph, which led to both
> > performance and memory usage issues.
> 
> Having a new class for each device would add a lot of classes.  Just how 
> badly this would affect lockdep's performance is hard to predict.  
> Testing seems like the best way to find out.

We support systems with 50000+ devices today, so one class per device
might be messy.

But back to the original issue here, why any of this?  What's wrong with
what we have now?  I haven't seen real locking issues reported yet (only
odd syzbot reports that didn't make any sense.)  Is this effort even
worth it?

thanks,

greg k-h