epoll: remove the on_list check for 'struct epitem'

By removing the 'int on_list' field from 'struct epitem', we avoid hitting
the BUILD_BUG_ON() for 'struct epitem' being larger than 128 bytes.

In file included from include/linux/init.h:4:0,
                 from fs/eventpoll.c:14:
fs/eventpoll.c: In function 'eventpoll_init':
include/linux/compiler.h:321:20: error: call to '__compiletime_assert_2137' declared with attribute error: BUILD_BUG_ON failed: sizeof(void *) <= 8 && sizeof(struct epitem) > 128
    prefix ## suffix();    \

The check to make sure that the 'struct epitem' was actually linked via
epi->fllink was added to avoid having the list removal primitives called
twice for the same 'struct epitem'.  However, the double call possibility
was removed by 'Subject: epoll: optimize EPOLL_CTL_DEL using rcu'.  There,
the call to 'list_del_init()' in eventpoll_release_file() was removed (we
now rely on the list delete happening entirely in 'ep_remove()', which is
called from eventpoll_release_file()).

There is also the question as to whether multiple ep_remove() calls could
happen concurrently.  This can not happen since EPOLL_CTL_DEL can't race
with eventpoll_release_file() or ep_free() - it has to do an fget() to
proceed.  Further, eventpoll_release_file() can not race with ep_free(),
since they both acquire the 'epmutex'.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Nathan Zimmer <nzimmer@sgi.com>
Cc: Eric Wong <normalperson@yhbt.net>
Cc: Nelson Elhage <nelhage@nelhage.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1830b1e897169cb77d88f01463bc96696a4ff5d3..584249454822f6950f512a05b2fe26945244b23a 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -170,9 +170,6 @@ struct epitem {
 
        /* The structure that describe the interested events and the source fd */
        struct epoll_event event;
-
-       /* The fllink is in use. Since rcu can't do 'list_del_init()' */
-       int on_list;
 };
 
 /*
@@ -706,10 +703,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 
        /* Remove the current item from the list of epoll hooks */
        spin_lock(&file->f_lock);
-       if (epi->on_list) {
-               list_del_rcu(&epi->fllink);
-               epi->on_list = 0;
-       }
+       list_del_rcu(&epi->fllink);
        spin_unlock(&file->f_lock);
 
        rb_erase(&epi->rbn, &ep->rbr);
@@ -1272,7 +1266,6 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
        epi->event = *event;
        epi->nwait = 0;
        epi->next = EP_UNACTIVE_PTR;
-       epi->on_list = 0;
        if (epi->event.events & EPOLLWAKEUP) {
                error = ep_create_wakeup_source(epi);
                if (error)
@@ -1306,7 +1299,6 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
        /* 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);
-       epi->on_list = 1;
        spin_unlock(&tfile->f_lock);
 
        /*
@@ -1347,8 +1339,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
 
 error_remove_epi:
        spin_lock(&tfile->f_lock);
-       if (epi->on_list)
-               list_del_rcu(&epi->fllink);
+       list_del_rcu(&epi->fllink);
        spin_unlock(&tfile->f_lock);
 
        rb_erase(&epi->rbn, &ep->rbr);