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

[Sergey Senozhatsky]

On (09/11/18 02:48), 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@xxxxxxxxxxxxxxxxxxxxxx>
> 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.

Certainly, something suspicious is going on.

> @@ -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);

Hmm. I think wake_up_process() executes a full memory barrier, because
it accesses task state.

>  		put_task_struct(tsk);
>  	}
> @@ -217,7 +219,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
>  	for (;;) {
>  		set_current_state(TASK_UNINTERRUPTIBLE);

I think that set_current_state() also executes memory barrier. Just
because it accesses task state.

> -		if (!waiter.task)
> +		if (!READ_ONCE(waiter.task))
>  			break;
>  		if (!timeout)
>  			break;

	-ss