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authorAl Viro <viro@zeniv.linux.org.uk>2020-10-02 03:45:51 +0300
committerAl Viro <viro@zeniv.linux.org.uk>2020-10-26 03:02:08 +0300
commit319c15174757aaedacc89a6e55c965416f130e64 (patch)
tree1dbdc3a651bb9a79cc3d23b7ba6bdc8c64470864 /include/linux/eventpoll.h
parentd9f41e3c95a17c263bd72799ea8c33ea6138dc22 (diff)
downloadlinux-319c15174757aaedacc89a6e55c965416f130e64.tar.xz
epoll: take epitem list out of struct file
Move the head of epitem list out of struct file; for epoll ones it's moved into struct eventpoll (->refs there), for non-epoll - into the new object (struct epitem_head). In place of ->f_ep_links we leave a pointer to the list head (->f_ep). ->f_ep is protected by ->f_lock and it's zeroed as soon as the list of epitems becomes empty (that can happen only in ep_remove() by now). The list of files for reverse path check is *not* going through struct file now - it's a single-linked list going through epitem_head instances. It's terminated by ERR_PTR(-1) (== EP_UNACTIVE_POINTER), so the elements of list can be distinguished by head->next != NULL. epitem_head instances are allocated at ep_insert() time (by attach_epitem()) and freed either by ep_remove() (if it empties the set of epitems *and* epitem_head does not belong to the reverse path check list) or by clear_tfile_check_list() when the list is emptied (if the set of epitems is empty by that point). Allocations are done from a separate slab - minimal kmalloc() size is too large on some architectures. As the result, we trim struct file _and_ get rid of the games with temporary file references. Locking and barriers are interesting (aren't they always); see unlist_file() and ep_remove() for details. The non-obvious part is that ep_remove() needs to decide if it will be the one to free the damn thing *before* actually storing NULL to head->epitems.first - that's what smp_load_acquire is for in there. unlist_file() lockless path is safe, since we hit it only if we observe NULL in head->epitems.first and whoever had done that store is guaranteed to have observed non-NULL in head->next. IOW, their last access had been the store of NULL into ->epitems.first and we can safely free the sucker. OTOH, we are under rcu_read_lock() and both epitem and epitem->file have their freeing RCU-delayed. So if we see non-NULL ->epitems.first, we can grab ->f_lock (all epitems in there share the same struct file) and safely recheck the emptiness of ->epitems; again, ->next is still non-NULL, so ep_remove() couldn't have freed head yet. ->f_lock serializes us wrt ep_remove(); the rest is trivial. Note that once head->epitems becomes NULL, nothing can get inserted into it - the only remaining reference to head after that point is from the reverse path check list. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'include/linux/eventpoll.h')
-rw-r--r--include/linux/eventpoll.h11
1 files changed, 1 insertions, 10 deletions
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index 4e215ccfa792..0350393465d4 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -22,14 +22,6 @@ struct file;
struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long toff);
#endif
-/* Used to initialize the epoll bits inside the "struct file" */
-static inline void eventpoll_init_file(struct file *file)
-{
- INIT_HLIST_HEAD(&file->f_ep_links);
- INIT_LIST_HEAD(&file->f_tfile_llink);
-}
-
-
/* Used to release the epoll bits inside the "struct file" */
void eventpoll_release_file(struct file *file);
@@ -50,7 +42,7 @@ static inline void eventpoll_release(struct file *file)
* because the file in on the way to be removed and nobody ( but
* eventpoll ) has still a reference to this file.
*/
- if (likely(hlist_empty(&file->f_ep_links)))
+ if (likely(!file->f_ep))
return;
/*
@@ -72,7 +64,6 @@ static inline int ep_op_has_event(int op)
#else
-static inline void eventpoll_init_file(struct file *file) {}
static inline void eventpoll_release(struct file *file) {}
#endif