linux-kernel - Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader

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Date:   Tue, 11 Sep 2018 13:40:43 +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>,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up
 reader

On Tue, Sep 11, 2018 at 02:48:17AM +0100, Dmitry Safonov wrote:
> There is a couple of reports about lockup in ldsem_down_read() without
> anyone holding write end of ldisc semaphore:
> lkml.kernel.org/r/<20171121132855.ajdv4k6swzhvktl6@...-t540p.sh.intel.com>
> lkml.kernel.org/r/<20180907045041.GF1110@...o2-debian>
> 
> They all looked like a missed wake up.
> I wasn't lucky enough to reproduce it, but it seems like reader on
> another CPU can miss waiter->task update and schedule again, resulting
> in indefinite (MAX_SCHEDULE_TIMEOUT) sleep.
> 
> Make sure waked up reader will see waiter->task == NULL.

> --- a/drivers/tty/tty_ldsem.c
> +++ b/drivers/tty/tty_ldsem.c
> @@ -118,6 +118,8 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)
>  		tsk = waiter->task;
>  		smp_mb();
>  		waiter->task = NULL;
> +		/* Make sure down_read_failed() will see !waiter->task update */
> +		smp_wmb();
>  		wake_up_process(tsk);

This is 'wrong', wake_up_process() should imply sufficient for this to
already be true. 

>  		put_task_struct(tsk);
>  	}