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From:   Jann Horn <jannh@google.com>
Date:   Thu, 27 Jul 2023 17:15:45 +0200
Message-ID: <CAG48ez1YPhc2sp=+pXCsZsmiPfXF_oQakouSfNFqi4xK2gEuGA@mail.gmail.com>
Subject: Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix
 anon_vma memory ordering
To:     Matthew Wilcox <willy@infradead.org>
Cc:     paulmck@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Suren Baghdasaryan <surenb@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>
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On Thu, Jul 27, 2023 at 5:07=E2=80=AFPM Matthew Wilcox <willy@infradead.org=
> wrote:
> On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote:
> > The other option is to replace the READ_ONCE() with a
> > smp_load_acquire(), at which point it becomes a lot simpler to show
> > that the code is correct.
>
> Aren't we straining at gnats here?  The context of this is handling a
> page fault, and we used to take an entire rwsem for read.  I'm having
> a hard time caring about "the extra expense" of an unnecessarily broad
> barrier.
>
> Cost of an L3 cacheline miss is in the thousands of cycles.  Cost of a
> barrier is ... tens?

Yeah, fair point. If it's hard to show correctness with READ_ONCE() we
can just use smp_load_acquire() and call it a day.