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

[Dmitry Safonov]

On Tue, 2018-09-11 at 13:40 +0200, Peter Zijlstra wrote:
> 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@xxxxxxxxxxxxxxxx
> > el.com>
> > lkml.kernel.org/r/<20180907045041.GF1110@shao2-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. 

Yeah, thanks.
It was stupid of me not to check that..
Saw the smoke that would describe the reports and made too long-going
conjectures. Need more covfefe and staring into that code.

> 
> >  		put_task_struct(tsk);
> >  	}

-- 
Thanks,
             Dmitry