linux-kernel - Re: [PATCH-tip v7 00/20] locking/rwsem: Rwsem rearchitecture part 2

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Date:   Sun, 28 Apr 2019 20:27:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        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>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH-tip v7 00/20] locking/rwsem: Rwsem rearchitecture part 2

On 4/28/19 8:10 PM, Linus Torvalds wrote:
> On Sun, Apr 28, 2019 at 4:12 PM Waiman Long <longman@...hat.com> wrote:
>> I implemented your suggestion in patch 1 as it will produce simpler and
>> faster code. However, one of the changes in my patchset is to wake up
>> all the readers in the wait list. This means I have to jump over the
>> writers and wake up the readers behind them as well. See patch 11 for
>> details. As a result, I have to revert back to use list_add_tail() and
>> list_for_each_entry_safe() for the first pass. That is why the diff for
>> the whole patchset is just the below change. It is done on purpose, not
>> an omission.
> Ahh, ok. In that case I suspect the clever code isn't even worth it,
> since it very much depends on just splitting the list in a fixed
> place.
>
>                    Linus

Not really, this is a serious problem that have to be backported to
earlier stable releases and downstream. The clever code is helpful in
those cases.

Cheers,
Longman