linux-kernel - Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted down

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Message-ID: <20180911135004.GU24106@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Sep 2018 15:50:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted
 down_read()

On Tue, Sep 11, 2018 at 02:33:22PM +0100, Dmitry Safonov wrote:
> > > You might want to think about ditching that ldsem thing entirely,
> > > and use a regular rwsem ?
> > 
> > Yeah, but AFAICS, regular rwsem will need to have a timeout then (for
> > write). So, I thought fixing this pile would be simpler than adding
> > timeout and probably writer-priority to generic rwsem?
> > 
> > And I guess, we still will need fixes for stable for the bugs here..
> > 
> > I expect that timeouts are ABI, while the gain of adding priority may
> > be measured. I'll give it a shot (adding timeout/priority for linux-
> > next) to rwsem if you say it's acceptable.
> 
> Actually, priority looks quite simple: we can add writers in the head
> of wait_list and it probably may work.
> Timeout looks also not a rocket science.
> So, I can try to do that if you say it's acceptable (with the gain
> measures).

So why do you need writer priority? The comment that goes with ldsems
doesn't explain I think, it just says it has it.

In general I dislike unfair locks, they always cause trouble.

> After this can of worms that I need to fix regardless.

Sure.