linux-kernel - Re: [PATCH 12/12] locking/rwsem: Make MSbit of count as guard bit to fail readlock

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Message-ID: <CAHk-=wi67qBi9d7zKfECs7ZnVJqbMoMx-ez-ohO39POqD9YR3Q@mail.gmail.com>
Date:   Thu, 28 Mar 2019 13:56:31 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH 12/12] locking/rwsem: Make MSbit of count as guard bit to
 fail readlock

On Thu, Mar 28, 2019 at 1:47 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Mar 28, 2019 at 11:12 AM Waiman Long <longman@...hat.com> wrote:
> >
> > With the merging of owner into count for x86-64, there is only 16 bits
> > left for reader count. It is theoretically possible for an application to
> > cause more than 64k readers to acquire a rwsem leading to count overflow.
>
> Ahh, and here's the thing that makes 16 bits work for readers.

Hmm. Does it?

Isn't there a race here? We're adding the READ bias, and then noticing
that it his the guard bit, and then the down_read_failed will make it
all good again.

But this isn't actually done with preemption disabled, so things
*could* get preempted in between, and if we have a huge run of bad
luck, it can still overflow.

Ok, so you need to have a 32k series run of bad luck (and hit
*exactly* the right small preemption point window every time), and I'm
certainly willing to say "yeah, not an issue", but maybe it's still
worth at least documenting?

                  Linus