linux-kernel - Re: [GIT PULL] RCU changes for v6.5

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Date:   Wed, 28 Jun 2023 10:34:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Zhu Yanjun <zyjzyj2000@...il.com>,
        Leon Romanovsky <leon@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-team@...a.com,
        mingo@...nel.org, tglx@...utronix.de, rcu@...r.kernel.org
Subject: Re: [GIT PULL] RCU changes for v6.5

On Tue, Jun 27, 2023 at 11:30:46AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 27, 2023 at 10:56:21AM -0700, Linus Torvalds wrote:
> > On Sun, 25 Jun 2023 at 08:35, Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > >   git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git tags/rcu.2023.06.22a
> > >
> > > o       Eliminate the single-argument variant of k[v]free_rcu() now
> > >         that all uses have been converted to k[v]free_rcu_mightsleep().
> > 
> > Well, clearly not all users had been.
> > 
> > The base of this RCU was v6.4-rc1, and when that commit was done, we
> > still had a single-argument variant:
> > 
> >   7e3f926bf453 ("rcu/kvfree: Eliminate k[v]free_rcu() single argument macro")
> > 
> > but look here:
> > 
> >      git grep 'kfree_rcu([^,()][^,()]*)' 7e3f926bf453
> > 
> > results in
> > 
> >    7e3f926bf453:drivers/infiniband/sw/rxe/rxe_verbs.c:     kfree_rcu(mr);
> > 
> > so the RCU tree itself can not possibly have built cleanly.
> > 
> > How the heck did this pass testing in linux-next? Did linux-next just
> > assume that it was a merge error, and fix it up?
> 
> Because idiot here failed to notice that the needed change was only
> in -next, and not yet in mainline.

It passed testing in linux-next because Stephen fixes eveything so it compiles:

https://lore.kernel.org/all/20230328121609.68105dd5@canb.auug.org.au/

It seems Stephen's report is a bit odd because it wasn't a merge issue
vs rdma, it was vs v6.4-rc1..

I suppose the question is why didn't something like Intel 0-day catch
it when it trial compiled the RCU tree's branch.

BTW, Stephen's merge is different than yours, his is based on Bob's
note here:

https://lore.kernel.org/linux-rdma/e704127e-1bfe-f351-db95-bfea6916e8f9@gmail.com/

I'll fix it in the next cycle.

Thanks,
Jason