linux-kernel - Re: Buggy rwsem locking code in fs/smb/client/file.c

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Message-ID: <CALF+zO=r8sV3nOOKSEmC_k5iFML-e_H9dUnKoK3fx7Z6DLz9Ag@mail.gmail.com>
Date:   Mon, 3 Jul 2023 14:46:19 -0400
From:   David Wysochanski <dwysocha@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        Pavel Shilovsky <pshilov@...rosoft.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ajay Kaher <akaher@...are.com>
Subject: Re: Buggy rwsem locking code in fs/smb/client/file.c

On Mon, Jul 3, 2023 at 1:00 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, 3 Jul 2023 at 08:43, Steven Rostedt <rostedt@...dmis.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.