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-=wh7eanC+W7DpSpAsAAYeRTp3EB5qUxMnDFW3HqNQC0GFQ@mail.gmail.com>
Date:   Thu, 28 Mar 2019 13:47:31 -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 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.

I think this at a minimum needs to be made very clear in the patch
that actually lowered the reader count bits, but preferably this just
needs to be done *before* that patch, so that you don't have a point
in history where rwlocks are simply not reliable.

               Linus