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

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Date:   Wed, 28 Jun 2023 06:46:50 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 Wed, Jun 28, 2023 at 10:34:34AM -0300, Jason Gunthorpe wrote:
> 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.

The revert meant that 0-day didn't see -rcu without the single-argument
kfree_rcu() definition.

							Thanx, Paul