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Message-Id: <20131030183241.AE8201FF9@prod-mail-relay06.akamai.com>
Date: Wed, 30 Oct 2013 18:32:41 +0000 (GMT)
From: Jason Baron <jbaron@...mai.com>
To: akpm@...ux-foundation.org
Cc: paulmck@...ux.vnet.ibm.com, normalperson@...t.net, nzimmer@....com,
viro@...iv.linux.org.uk, nelhage@...hage.com,
davidel@...ilserver.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: [PATCH] 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@...mai.com>
---
fs/eventpoll.c | 13 ++-----------
1 file changed, 2 insertions(+), 11 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 755bda0..69de7a6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -171,9 +171,6 @@ 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;
};
/*
@@ -710,10 +707,7 @@ 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);
@@ -1295,7 +1289,6 @@ 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)
@@ -1329,7 +1322,6 @@ 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);
/*
@@ -1370,8 +1362,7 @@ 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);
--
1.8.2
--
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