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

Sergey Senozhatsky
2018-09-11T05:04:57Z

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@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.

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