On Fri, 1 Nov 2013 10:09:12 +1100 Stephen Rothwell <[email protected]> wrote:
> Hi Andrew, > > On Wed, 30 Oct 2013 18:32:41 +0000 (GMT) Jason Baron <[email protected]> > wrote: > > > > 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 <[email protected]> > > Do you want me to put this in my copy of the mmotm instead of reverting > these three? > > epoll-do-not-take-global-epmutex-for-simple-topologies-fix > epoll: do not take global 'epmutex' for simple topologies > epoll: optimize EPOLL_CTL_DEL using rcu Sure. Here's my epoll-optimize-epoll_ctl_del-using-rcu-fix.patch: From: Jason Baron <[email protected]> Subject: 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 <[email protected]> Reported-by: Wu Fengguang <[email protected]> Cc: Nathan Zimmer <[email protected]> Cc: Eric Wong <[email protected]> Cc: Nelson Elhage <[email protected]> Cc: Al Viro <[email protected]> Cc: Davide Libenzi <[email protected]> Cc: "Paul E. McKenney" <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- fs/eventpoll.c | 13 ++----------- 1 file changed, 2 insertions(+), 11 deletions(-) diff -puN fs/eventpoll.c~epoll-optimize-epoll_ctl_del-using-rcu-fix fs/eventpoll.c --- a/fs/eventpoll.c~epoll-optimize-epoll_ctl_del-using-rcu-fix +++ a/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; }; /* @@ -707,10 +704,7 @@ static int ep_remove(struct eventpoll *e /* 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); @@ -1273,7 +1267,6 @@ static int ep_insert(struct eventpoll *e 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) @@ -1307,7 +1300,6 @@ static int ep_insert(struct eventpoll *e /* 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); /* @@ -1348,8 +1340,7 @@ static int ep_insert(struct eventpoll *e 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); _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
