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

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Message-ID: <1536672802.2710.33.camel@arista.com>
Date:   Tue, 11 Sep 2018 14:33:22 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Peter Zijlstra <peterz@...radead.org>
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, 2018-09-11 at 14:01 +0100, Dmitry Safonov wrote:
> On Tue, 2018-09-11 at 14:02 +0200, Peter Zijlstra wrote:
> > On Tue, Sep 11, 2018 at 02:48:21AM +0100, Dmitry Safonov wrote:
> > > It seems like when ldsem_down_read() fails with timeout, it
> > > misses
> > > update for sem->wait_readers. By that reason, when writer finally
> > > releases write end of the semaphore __ldsem_wake_readers() does
> > > adjust
> > > sem->count with wrong value:
> > > sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS)
> > > 
> > > I.e, if update comes with 1 missed wait_readers decrement, sem-
> > > > count
> > > 
> > > will be 0x100000001 which means that there is active reader and
> > > it'll
> > > make any further writer to fail in acquiring the semaphore.
> > > 
> > > It looks like, this is a dead-code, because ldsem_down_read() is
> > > never
> > > called with timeout different than MAX_SCHEDULE_TIMEOUT, so it
> > > might be
> > > worth to delete timeout parameter and error path fall-back..
> > 
> > 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).

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

-- 
Thanks,
             Dmitry