On 2020-05-02 00:09, Jason Baron wrote:
On 5/1/20 5:02 PM, Roman Penyaev wrote:
Hi Jason,
That is indeed a nice catch.
Seems we need smp_rmb() pair between list_empty_careful(&rp->rdllist)
and
READ_ONCE(ep->ovflist) for ep_events_available(), do we?
Hi Roman,
Good point, even if we order those reads its still racy, since the
read of the ready list could come after its been cleared and the
read of the overflow could again come after its been cleared.
You mean the second chunk? True. Sigh.
So I'm afraid we might need instead something like this to make
sure they are read together:
No, impossible, I can't believe in that :) We can't give up.
All we need is to keep a mark, that ep->rdllist is not empty,
even we've just spliced it. ep_poll_callback() always takes
the ->ovflist path, if ->ovflist is not EP_UNACTIVE_PTR, but
ep_events_available() does not need to observe ->ovflist at
all, just a ->rdllist.
Take a look at that, do I miss something? :
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aba03ee749f8..a8770f9a917e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -376,8 +376,7 @@ static void ep_nested_calls_init(struct nested_calls
*ncalls)
*/
static inline int ep_events_available(struct eventpoll *ep)
{
- return !list_empty_careful(&ep->rdllist) ||
- READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
+ return !list_empty_careful(&ep->rdllist);
}
#ifdef CONFIG_NET_RX_BUSY_POLL
@@ -683,7 +682,8 @@ static __poll_t ep_scan_ready_list(struct eventpoll
*ep,
{
__poll_t res;
struct epitem *epi, *nepi;
- LIST_HEAD(txlist);
+ LIST_HEAD(rdllist);
+ LIST_HEAD(ovflist);
lockdep_assert_irqs_enabled();
@@ -704,14 +704,22 @@ static __poll_t ep_scan_ready_list(struct
eventpoll *ep,
* in a lockless way.
*/
write_lock_irq(&ep->lock);
- list_splice_init(&ep->rdllist, &txlist);
+ /*
+ * We do not call list_splice_init() because for lockless
+ * ep_events_available() ->rdllist is still "not empty".
+ * Otherwise the feature that there is something left in
+ * the list can be lost which causes missed wakeup.
+ */
+ list_splice(&ep->rdllist, &rdllist);
+ /*
+ * If ->rdllist was empty we should pretend it was not,
+ * because after the unlock ->ovflist comes into play,
+ * which is invisible for lockless ep_events_available().
+ */
+ ep->rdllist.next = LIST_POISON1;
WRITE_ONCE(ep->ovflist, NULL);
write_unlock_irq(&ep->lock);
/*
* Now call the callback function.
*/
- res = (*sproc)(ep, &txlist, priv);
+ res = (*sproc)(ep, &rdllist, priv);
write_lock_irq(&ep->lock);
/*
@@ -724,7 +732,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll
*ep,
/*
* We need to check if the item is already in the list.
* During the "sproc" callback execution time, items are
- * queued into ->ovflist but the "txlist" might already
+ * queued into ->ovflist but the "rdllist" might already
* contain them, and the list_splice() below takes care
of them.
*/
if (!ep_is_linked(epi)) {
@@ -732,7 +740,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll
*ep,
* ->ovflist is LIFO, so we have to reverse it
in order
* to keep in FIFO.
*/
- list_add(&epi->rdllink, &ep->rdllist);
+ list_add(&epi->rdllink, &ovflist);
ep_pm_stay_awake(epi);
}
}
@@ -743,10 +751,11 @@ static __poll_t ep_scan_ready_list(struct
eventpoll *ep,
*/
WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);
- /*
- * Quickly re-inject items left on "txlist".
- */
- list_splice(&txlist, &ep->rdllist);
+ /* Events from ->ovflist happened later, thus splice to the tail
*/
+ list_splice_tail(&ovflist, &rdllist);
+ /* Just replace list */
+ list_replace(&rdllist, &ep->rdllist);
+
__pm_relax(ep->ws);
write_unlock_irq(&ep->lock);
@@ -1763,13 +1772,13 @@ static __poll_t ep_send_events_proc(struct
eventpoll *ep, struct list_head *head
* Trigger mode, we need to insert back inside
* the ready list, so that the next call to
* epoll_wait() will check again the events
- * availability. At this point, no one can
insert
- * into ep->rdllist besides us. The epoll_ctl()
- * callers are locked out by
- * ep_scan_ready_list() holding "mtx" and the
- * poll callback will queue them in ep->ovflist.
+ * availability. What we do here is simply
+ * return the epi to the same position where
+ * it was, the ep_scan_ready_list() will
+ * re-inject the leftovers to the ->rdllist
+ * under the proper lock.
*/
- list_add_tail(&epi->rdllink, &ep->rdllist);
+ list_add_tail(&epi->rdllink, &tmp->rdllink);
ep_pm_stay_awake(epi);
}
}
--
Roman
