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

[Peter Zijlstra <peterz@infradead.org>]

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@wfg-t540p.sh.intel.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. 

>  		put_task_struct(tsk);
>  	}