[PATCH v4 09/14] epoll: support polling from userspace for ep_remove()

[Roman Penyaev]

On ep_remove() simply mark a user item with EPOLLREMOVE if the item was
ready (i.e. has some bits set).  That will prevent further user index
entry creation on item ->bit reuse.

Signed-off-by: Roman Penyaev <rpeny...@suse.de>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: linux-fsde...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 fs/eventpoll.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 4f541f85c7e5..edf7ba28bce0 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -879,12 +879,30 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
        return res;
 }
 
-static void epi_rcu_free(struct rcu_head *head)
+static void epi_rcu_free_cb(struct rcu_head *head)
 {
        struct epitem *epi = container_of(head, struct epitem, rcu);
        kmem_cache_free(epi_cache, epi);
 }
 
+static void uepi_rcu_free_cb(struct rcu_head *head)
+{
+       struct epitem *epi = container_of(head, struct epitem, rcu);
+       kmem_cache_free(uepi_cache, uep_item_from_epi(epi));
+}
+
+static void epi_rcu_free(struct eventpoll *ep, struct epitem *epi)
+{
+       /*
+        * Check if `ep` is polled by user here, in this function, not
+        * in the callback, in order not to control lifetime of the 'ep'.
+        */
+       if (ep_polled_by_user(ep))
+               call_rcu(&epi->rcu, uepi_rcu_free_cb);
+       else
+               call_rcu(&epi->rcu, epi_rcu_free_cb);
+}
+
 static inline int ep_get_bit(struct eventpoll *ep)
 {
        bool was_set;
@@ -1156,10 +1174,14 @@ static int ep_remove(struct eventpoll *ep, struct 
epitem *epi)
 
        rb_erase_cached(&epi->rbn, &ep->rbr);
 
-       write_lock_irq(&ep->lock);
-       if (ep_is_linked(epi))
-               list_del_init(&epi->rdllink);
-       write_unlock_irq(&ep->lock);
+       if (ep_polled_by_user(ep)) {
+               ep_remove_user_item(epi);
+       } else {
+               write_lock_irq(&ep->lock);
+               if (ep_is_linked(epi))
+                       list_del_init(&epi->rdllink);
+               write_unlock_irq(&ep->lock);
+       }
 
        wakeup_source_unregister(ep_wakeup_source(epi));
        /*
@@ -1169,7 +1191,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem 
*epi)
         * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
         * use of the rbn field.
         */
-       call_rcu(&epi->rcu, epi_rcu_free);
+       epi_rcu_free(ep, epi);
 
        atomic_long_dec(&ep->user->epoll_watches);
 
-- 
2.21.0