Re: [PATCH] epoll: fix sparse error on RCU assignment

[Oleg Nesterov <oleg@redhat.com>]

On 03/10, Eric Wong wrote:
>
> This fixes the following sparse error when using
> CONFIG_SPARSE_RCU_POINTER=y and "make C=2 fs/eventpoll.o"
>
>   fs/eventpoll.c:514:17: error: incompatible types in comparison expression (different address spaces)

ep_remove_wait_queue() does rcu_dereference(pwq->whead) and
rcu_dereference_sparse(__rcu) complains, I guess.

> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -228,7 +228,7 @@ struct eppoll_entry {
>  	wait_queue_t wait;
>
>  	/* The wait queue head that linked the "wait" wait queue item */
> -	wait_queue_head_t *whead;
> +	wait_queue_head_t __rcu *whead;

Well, perhaps this change is fine... but otoh this this a bit misleading.
It is not actually __rcu. The special case is sighand->signalfd_wqh, and
the commemt in ep_remove_wait_queue() means: if ->whead is not stable then
we can only race with signalfd_cleanup(), and rcu_read_lock() ensures this
memory can't go away.

We do not even need smp_read_barrier_depends() here, ACCESS_ONCE() should
be enough.

Perhaps it would be better to simply shut up this warning somehow...

>  };
>
>  /* Wrapper struct used by poll queueing */
> @@ -929,7 +929,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
>  	struct eventpoll *ep = epi->ep;
>
>  	if ((unsigned long)key & POLLFREE) {
> -		ep_pwq_from_wait(wait)->whead = NULL;
> +		RCU_INIT_POINTER(ep_pwq_from_wait(wait)->whead, NULL);
>  		/*
>  		 * whead = NULL above can race with ep_remove_wait_queue()
>  		 * which can do another remove_wait_queue() after us, so we
> @@ -1018,7 +1018,7 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
>
>  	if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
>  		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
> -		pwq->whead = whead;
> +		RCU_INIT_POINTER(pwq->whead, whead);
>  		pwq->base = epi;
>  		add_wait_queue(whead, &pwq->wait);
>  		list_add_tail(&pwq->llink, &epi->pwqlist);
> -- 
> Eric Wong