Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable

[Michal Hocko]

On Wed 11-05-16 10:35:12, Peter Zijlstra wrote:
> On Wed, May 11, 2016 at 09:23:57AM +0200, Michal Hocko wrote:
> > On Tue 10-05-16 14:38:06, Peter Zijlstra wrote:
> 
> > > Also, looking at it again; I think we're forgetting to re-adjust the
> > > BIAS for the cancelled writer.
> > 
> > Hmm, __rwsem_down_write_failed_common does
> > 
> > 	/* undo write bias from down_write operation, stop active locking */
> > 	count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
> > 
> > which should remove the bias AFAIU.
> 
> Right; at this point we're neutral wrt bias.
> 
> > Later we do
> > 
> > 	if (waiting) {
> > 		count = READ_ONCE(sem->count);
> > 
> > 		/*
> > 		 * If there were already threads queued before us and there are
> > 		 * no active writers, the lock must be read owned; so we try to
> > 		 * wake any read locks that were queued ahead of us.
> > 		 */
> > 		if (count > RWSEM_WAITING_BIAS)
> > 			sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
> > 
> > 	} else
> > 		count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> > 
> > and that might set RWSEM_WAITING_BIAS but the current holder of the lock
> > should handle that correctly and wake the waiting tasks IIUC. I will go
> > and check the code closer. It is quite easy to get this subtle code
> > wrong..
> 
> Subtle; yes.
> 
> So if you look at rwsem_try_write_lock() -- traditionally the only way
> to exit this wait loop, you see it does:
> 
> 	if (count == RWSEM_WAITING_BIAS &&
> 	    cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
> 		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
> 		if (!list_is_singular(&sem->wait_list))
> 			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> 		rwsem_set_owner(sem);
> 		return true;
> 	}
> 
> Which ends up clearing RWSEM_WAITING_BIAS is we were the only waiter --
> or rather, it always clear WAITING, but then tests the list and re-sets
> it if there's more than one waiters on.
> 
> Now, the signal break doesn't clear WAITING if we were the only waiter
> on the list; which means any further down_read() will block (I didn't
> look at what a subsequent down_write() would do).

I was staring at this part as well but then I convinced myself that this
is OK because rwsem_down_read_failed does:

	if (list_empty(&sem->wait_list))
		adjustment += RWSEM_WAITING_BIAS;
	list_add_tail(&waiter.list, &sem->wait_list);

	/* we're now waiting on the lock, but no longer actively locking */
	count = rwsem_atomic_update(adjustment, sem);

	/* If there are no active locks, wake the front queued process(es).
	 *
	 * If there are no writers and we are first in the queue,
	 * wake our own waiter to join the existing active readers !
	 */
	if (count == RWSEM_WAITING_BIAS ||
	    (count > RWSEM_WAITING_BIAS &&
	     adjustment != -RWSEM_ACTIVE_READ_BIAS))
		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);

__rwsem_do_wake should then see all the readers (including the current
one) and wake them up and set waiter->task to NULL to allow the current
one to break out of the loop as well.

> So I think we needs something like this, to clear WAITING if we leave
> the list empty.

But maybe this is the correct way to go.

> Does that make sense?

I do not see an immediate problem with this. Maybe Tetsuo can try this
out and see if it really makes a difference.

> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index df4dcb883b50..7011dd1c286c 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -489,6 +489,8 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
>  		do {
>  			if (signal_pending_state(state, current)) {
>  				raw_spin_lock_irq(&sem->wait_lock);
> +				if (list_singular(&sem->wait_list))
> +					rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem);
>  				ret = ERR_PTR(-EINTR);
>  				goto out;
>  			}

-- 
Michal Hocko
SUSE Labs