Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230703114318.1576ea24@rorschach.local.home> <CAHk-=wg4J3XjdakYiK6yC-QuBuCDn1g4XGNkuaAzByUM4KCCog@mail.gmail.com>
In-Reply-To: <CAHk-=wg4J3XjdakYiK6yC-QuBuCDn1g4XGNkuaAzByUM4KCCog@mail.gmail.com>
From:   David Wysochanski <dwysocha@redhat.com>
Date:   Mon, 3 Jul 2023 14:46:19 -0400
Message-ID: <CALF+zO=r8sV3nOOKSEmC_k5iFML-e_H9dUnKoK3fx7Z6DLz9Ag@mail.gmail.com>
Subject: Re: Buggy rwsem locking code in fs/smb/client/file.c
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ronnie Sahlberg <lsahlber@redhat.com>,
        Pavel Shilovsky <pshilov@microsoft.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ajay Kaher <akaher@vmware.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Mon, Jul 3, 2023 at 1:00=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, 3 Jul 2023 at 08:43, Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > And hides the bug by wrapping the down_write() with:
> >
> > +void
> > +cifs_down_write(struct rw_semaphore *sem)
> > +{
> > +       while (!down_write_trylock(sem))
> > +               msleep(10);
> > +}
>
> That is indeed disgusting.
>
> It may *work* - because as the commit message says, it means that
> writers are now never queued up and thus never block recursive
> readers.
>
> And in the process it now becomes absolutely horribly unfair to
> writers, who will easily get starved by readers.
>
> This is absolutely not acceptable in any sane situation. Are writers
> *so* rare and special that starving them is ok?
>
> Because starvation can be just as deadly as a deadlock. You're just
> hiding the problem from lockdep and yourself.
>
> This is very much a "head in the sand" solution.
>
>             Linus
>

Steve and Linus,

Thank you for pointing this out.  I'll have to get with Ronnie on this
and come up with something better.