linux-kernel - Re: [PATCH 10/12] locking/rwsem: Merge owner into count on x86-64

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Date:   Thu, 28 Mar 2019 13:43:58 -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 10/12] locking/rwsem: Merge owner into count on x86-64

On Thu, Mar 28, 2019 at 11:12 AM Waiman Long <longman@...hat.com> wrote:
>
>  Reserving 2 bits for status flags,
> we will have 16 bits for the reader count.  That can supports up to
> (64k-1) readers.

Explain why that's enough, please.

I could *easily* see more than 64k threads all on the same rwsem, all
at the same time.

Just do a really slow filesystem (think fuse), map a file with lots of
pages, and then fault in one page per thread. Boom. rwsem with more
than 64k concurrent readers.

So I think this approach is completely wrong, and/or needs a *lot* of
explanation why it works.

A small reader count works for the spinning rwlocks because we're
limited to the number of CPU's in the system. For a rwsem? No.

                    Linus