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SubjectRe: [PATCH 4/4] epoll: ep_unregister_pollwait() can use the freed pwq->whead
On 02/22, Oleg Nesterov wrote:
>
> However this needs more locking. ep_remove_wait_queue() should take
> ep->lock first to avoid the race and pin pwq->whead, then it needs
> pwq->whead->lock for __remove_wait_queue().
>
> This can obviously AB-BA deadlock with wake_up()->ep_poll_callback(),
> so ep_remove_wait_queue() does the nasty lock + trylock-or-retry dance.

Or we can rely on the fact that sighand_cachep is SLAB_DESTROY_BY_RCU,
and assume that ->whead is always rcu-protected if it can go away.

In this case we don't need 3/4 (although it makes sense to add the
fat comment), and 4/4 can be simplified, see below.

ep_pwq_from_wait() is not really needed, it has a single caller.
Just I tried to follow the coding style.

I'd prefer the explicit locking, but I don't really mind.

Oleg.

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -320,6 +320,11 @@ static inline int ep_is_linked(struct list_head *p)
return !list_empty(p);
}

+static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)
+{
+ return container_of(p, struct eppoll_entry, wait);
+}
+
/* Get the "struct epitem" from a wait queue pointer */
static inline struct epitem *ep_item_from_wait(wait_queue_t *p)
{
@@ -467,6 +472,17 @@ static void ep_poll_safewake(wait_queue_head_t *wq)
put_cpu();
}

+static void ep_remove_wait_queue(struct eppoll_entry *pwq)
+{
+ wait_queue_head_t *whead;
+
+ rcu_read_lock();
+ whead = rcu_dereference(pwq->whead);
+ if (whead)
+ remove_wait_queue(whead, &pwq->wait);
+ rcu_read_unlock();
+}
+
/*
* This function unregisters poll callbacks from the associated file
* descriptor. Must be called with "mtx" held (or "epmutex" if called from
@@ -481,7 +497,7 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
pwq = list_first_entry(lsthead, struct eppoll_entry, llink);

list_del(&pwq->llink);
- remove_wait_queue(pwq->whead, &pwq->wait);
+ ep_remove_wait_queue(pwq);
kmem_cache_free(pwq_cache, pwq);
}
}
@@ -845,8 +861,11 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
spin_lock_irqsave(&ep->lock, flags);

/* the caller holds eppoll_entry->whead->lock */
- if ((unsigned long)key & POLLFREE)
+ if ((unsigned long)key & POLLFREE) {
+ /* can't use __remove_wait_queue(), we need list_del_init() */
list_del_init(&wait->task_list);
+ ep_pwq_from_wait(wait)->whead = NULL;
+ }

/*
* If the event mask does not contain any poll(2) event, we consider the


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