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ep_insert(): we only need tep->mtx around the insertion itself 

We do need ep->mtx (and we are holding it all along), but that's
the lock on the epoll we are inserting into; locking of the
epoll being inserted is not needed for most of that work -
as the matter of fact, we only need it to provide barriers
for the fastpath check (for now).

Move taking and releasing it into ep_insert().  The caller
(do_epoll_ctl()) doesn't need to bother with that at all.
Moreover, that way we kill the kludge in ep_item_poll() - now
it's always called with tep unlocked.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2020-09-26 18:15:26 -04:00
parent e3e096e7fc
commit 85353e919f
1 changed files with 10 additions and 18 deletions
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28
fs/eventpoll.c
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@ -731,8 +731,6 @@ static int ep_eventpoll_release(struct inode *inode, struct file *file)
static __poll_t ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
int depth);
static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
poll_table *pt);
/*
* Differs from ep_eventpoll_poll() in that internal callers already have
@ -745,7 +743,6 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
struct eventpoll *ep;
LIST_HEAD(txlist);
__poll_t res;
bool locked;
pt->_key = epi->event.events;
if (!is_file_epoll(epi->ffd.file))
@ -754,15 +751,11 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
ep = epi->ffd.file->private_data;
poll_wait(epi->ffd.file, &ep->poll_wait, pt);
// kludge: ep_insert() calls us with ep->mtx already locked
locked = pt && (pt->_qproc == ep_ptable_queue_proc);
if (!locked)
mutex_lock_nested(&ep->mtx, depth);
mutex_lock_nested(&ep->mtx, depth);
ep_start_scan(ep, &txlist);
res = ep_read_events_proc(ep, &txlist, depth + 1);
ep_done_scan(ep, &txlist);
if (!locked)
mutex_unlock(&ep->mtx);
mutex_unlock(&ep->mtx);
return res & epi->event.events;
}
@ -1365,6 +1358,10 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
long user_watches;
struct epitem *epi;
struct ep_pqueue epq;
struct eventpoll *tep = NULL;
if (is_file_epoll(tfile))
tep = tfile->private_data;
lockdep_assert_irqs_enabled();
@ -1394,6 +1391,8 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
atomic_long_inc(&ep->user->epoll_watches);
if (tep)
mutex_lock_nested(&tep->mtx, 1);
/* Add the current item to the list of active epoll hook for this file */
spin_lock(&tfile->f_lock);
list_add_tail_rcu(&epi->fllink, &tfile->f_ep_links);
@ -1404,6 +1403,8 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
* protected by "mtx", and ep_insert() is called with "mtx" held.
*/
ep_rbtree_insert(ep, epi);
if (tep)
mutex_unlock(&tep->mtx);
/* now check if we've created too many backpaths */
if (unlikely(full_check && reverse_path_check())) {
@ -2034,13 +2035,6 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
if (error)
goto error_tgt_fput;
if (is_file_epoll(tf.file)) {
error = epoll_mutex_lock(&tep->mtx, 1, nonblock);
if (error) {
mutex_unlock(&ep->mtx);
goto error_tgt_fput;
}
}
}
}
@ -2076,8 +2070,6 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
error = -ENOENT;
break;
}
if (tep != NULL)
mutex_unlock(&tep->mtx);
mutex_unlock(&ep->mtx);
error_tgt_fput: