linux-kernel - Re: [GIT PULL] RCU regression fix for v6.4

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Date:   Fri, 16 Jun 2023 14:26:59 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rcu@...r.kernel.org, jonathanh@...dia.com, wenst@...omium.org,
        angelogioacchino.delregno@...labora.com,
        rafael.j.wysocki@...el.com, mirq-linux@...e.qmqm.pl,
        dmitry.osipenko@...labora.com, sachinp@...ux.ibm.com,
        qiang.zhang1211@...il.com
Subject: Re: [GIT PULL] RCU regression fix for v6.4

On Fri, Jun 16, 2023 at 11:50:31AM -0700, Linus Torvalds wrote:
> On Fri, 16 Jun 2023 at 09:34, Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > Yes, it would be nice to abstract this somehow in order to hide it in
> > SRCU, but I still don't see a good way of doing this.
> 
> Ehh - like we actually do spinlocks in general, perhaps?
> 
> Spinlocks always exist. On UP - with no spinlock debugging - it turns
> into a zero-sized data structure, and the spin lock/unlock operations
> are no-ops.
> 
> Why don't you just do the exact same thing with the "struct
> srcu_usage". For Tiny SRCU, just make it an empty struct, with an
> empty initializer.
> 
> IOW, don't "abstract it out". Abstract it IN. Make tiny-rcu still have
> it, just as a no-op.
> 
> Anyway, I've pulled your fix, but maybe the above would have been the
> cleaner solution?

Good point, thank you!  I will add the lock to Tiny SRCU, shooting for
the v6.6 merge window.

							Thanx, Paul