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

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Message-ID: <20180911120258.GC19234@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Sep 2018 14:02:58 +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 02:48:21AM +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 ?