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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 4 Dec 2020 09:21:49 -0800
Message-ID: <CAHk-=wi6inOF5yvQRwUFbqMt0zFJ8S8GhqE2M0judU7RiGru8Q@mail.gmail.com>
Subject: Re: [PATCH 3/3] exec: Transform exec_update_mutex into a rw_semaphore
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Jann Horn <jannh@google.com>,
        Vasiliy Kulikov <segoon@openwall.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Oleg Nesterov <oleg@redhat.com>,
        Christopher Yeoh <cyeoh@au1.ibm.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>
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On Fri, Dec 4, 2020 at 8:08 AM Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
>
> >
> > -static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
> > +static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
> >  {
> > -     if (likely(m2 != m1))
> > -             mutex_unlock(m2);
> > -     mutex_unlock(m1);
> > +     if (likely(l2 != l1))
>
> is this still necessary ?
>
> > +             up_read(l2);
> > +     up_read(l1);
> >  }
> >
> > -static int kcmp_lock(struct mutex *m1, struct mutex *m2)
> > +static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
> >  {
> >       int err;
> >
> > -     if (m2 > m1)
> > -             swap(m1, m2);
> > +     if (l2 > l1)
> > +             swap(l1, l2);
>
> and this is probably also no longer necessary?

These are still necessary, because even a recursive read lock can
still block on a writer trying to come in between the two read locks
due to fairness guarantees.

So taking the same read lock twice is still a source of possible deadlocks.

For the same reason, read locks still have ABBA deadlock and need to
be taken in order.

So switching from a mutex to a rwlock doesn't really change the
locking rules in this respect.

In fact, I'm not convinced this change even fixes the deadlock that
syzbot reported, for the same reason: it just requires a write lock in
between two read locks to deadlock.

               Linus