From c7ea76302547f81e4583d0d7c52a1c37c6747f5d Mon Sep 17 00:00:00 2001 From: Davide Libenzi Date: Tue, 15 May 2007 01:40:47 -0700 Subject: [PATCH] epoll locks changes and cleanups Changes the rwlock to a spinlock, and drops the use-count variable. Operations are always bound by the mutex now, so the use-count is no more needed. For the same reason, the rwlock can become a simple spinlock. Signed-off-by: Davide Libenzi Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/eventpoll.c | 232 +++++++++++++++---------------------------------- 1 file changed, 72 insertions(+), 160 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 1dbedc71a28..4c16127c96b 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1,6 +1,6 @@ /* - * fs/eventpoll.c ( Efficent event polling implementation ) - * Copyright (C) 2001,...,2006 Davide Libenzi + * fs/eventpoll.c (Efficent event polling implementation) + * Copyright (C) 2001,...,2007 Davide Libenzi * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by @@ -44,8 +44,8 @@ * There are three level of locking required by epoll : * * 1) epmutex (mutex) - * 2) ep->mtx (mutes) - * 3) ep->lock (rw_lock) + * 2) ep->mtx (mutex) + * 3) ep->lock (spinlock) * * The acquire order is the one listed above, from 1 to 3. * We need a spinlock (ep->lock) because we manipulate objects @@ -140,6 +140,12 @@ struct epitem { /* List header used to link this structure to the eventpoll ready list */ struct list_head rdllink; + /* + * Works together "struct eventpoll"->ovflist in keeping the + * single linked chain of items. + */ + struct epitem *next; + /* The file descriptor information this item refers to */ struct epoll_filefd ffd; @@ -152,23 +158,11 @@ struct epitem { /* The "container" of this item */ struct eventpoll *ep; - /* The structure that describe the interested events and the source fd */ - struct epoll_event event; - - /* - * Used to keep track of the usage count of the structure. This avoids - * that the structure will desappear from underneath our processing. - */ - atomic_t usecnt; - /* List header used to link this item to the "struct file" items list */ struct list_head fllink; - /* - * Works together "struct eventpoll"->ovflist in keeping the - * single linked chain of items. - */ - struct epitem *next; + /* The structure that describe the interested events and the source fd */ + struct epoll_event event; }; /* @@ -178,7 +172,7 @@ struct epitem { */ struct eventpoll { /* Protect the this structure access */ - rwlock_t lock; + spinlock_t lock; /* * This mutex is used to ensure that files are not removed @@ -393,79 +387,12 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi) } } -/* - * Unlink the "struct epitem" from all places it might have been hooked up. - * This function must be called with write IRQ lock on "ep->lock". - */ -static int ep_unlink(struct eventpoll *ep, struct epitem *epi) -{ - int error; - - /* - * It can happen that this one is called for an item already unlinked. - * The check protect us from doing a double unlink ( crash ). - */ - error = -ENOENT; - if (!ep_rb_linked(&epi->rbn)) - goto error_return; - - /* - * Clear the event mask for the unlinked item. This will avoid item - * notifications to be sent after the unlink operation from inside - * the kernel->userspace event transfer loop. - */ - epi->event.events = 0; - - /* - * At this point is safe to do the job, unlink the item from our rb-tree. - * This operation togheter with the above check closes the door to - * double unlinks. - */ - ep_rb_erase(&epi->rbn, &ep->rbr); - - /* - * If the item we are going to remove is inside the ready file descriptors - * we want to remove it from this list to avoid stale events. - */ - if (ep_is_linked(&epi->rdllink)) - list_del_init(&epi->rdllink); - - error = 0; -error_return: - - DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_unlink(%p, %p) = %d\n", - current, ep, epi->ffd.file, error)); - - return error; -} - -/* - * Increment the usage count of the "struct epitem" making it sure - * that the user will have a valid pointer to reference. - */ -static void ep_use_epitem(struct epitem *epi) -{ - atomic_inc(&epi->usecnt); -} - -/* - * Decrement ( release ) the usage count by signaling that the user - * has finished using the structure. It might lead to freeing the - * structure itself if the count goes to zero. - */ -static void ep_release_epitem(struct epitem *epi) -{ - if (atomic_dec_and_test(&epi->usecnt)) - kmem_cache_free(epi_cache, epi); -} - /* * Removes a "struct epitem" from the eventpoll RB tree and deallocates - * all the associated resources. + * all the associated resources. Must be called with "mtx" held. */ static int ep_remove(struct eventpoll *ep, struct epitem *epi) { - int error; unsigned long flags; struct file *file = epi->ffd.file; @@ -485,26 +412,21 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi) list_del_init(&epi->fllink); spin_unlock(&file->f_ep_lock); - /* We need to acquire the write IRQ lock before calling ep_unlink() */ - write_lock_irqsave(&ep->lock, flags); + if (ep_rb_linked(&epi->rbn)) + ep_rb_erase(&epi->rbn, &ep->rbr); - /* Really unlink the item from the RB tree */ - error = ep_unlink(ep, epi); - - write_unlock_irqrestore(&ep->lock, flags); - - if (error) - goto error_return; + spin_lock_irqsave(&ep->lock, flags); + if (ep_is_linked(&epi->rdllink)) + list_del_init(&epi->rdllink); + spin_unlock_irqrestore(&ep->lock, flags); /* At this point it is safe to free the eventpoll item */ - ep_release_epitem(epi); + kmem_cache_free(epi_cache, epi); - error = 0; -error_return: - DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_remove(%p, %p) = %d\n", - current, ep, file, error)); + DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_remove(%p, %p)\n", + current, ep, file)); - return error; + return 0; } static void ep_free(struct eventpoll *ep) @@ -574,10 +496,10 @@ static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait) poll_wait(file, &ep->poll_wait, wait); /* Check our condition */ - read_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); if (!list_empty(&ep->rdllist)) pollflags = POLLIN | POLLRDNORM; - read_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); return pollflags; } @@ -636,7 +558,7 @@ static int ep_alloc(struct eventpoll **pep) if (!ep) return -ENOMEM; - rwlock_init(&ep->lock); + spin_lock_init(&ep->lock); mutex_init(&ep->mtx); init_waitqueue_head(&ep->wq); init_waitqueue_head(&ep->poll_wait); @@ -652,20 +574,18 @@ static int ep_alloc(struct eventpoll **pep) } /* - * Search the file inside the eventpoll tree. It add usage count to - * the returned item, so the caller must call ep_release_epitem() - * after finished using the "struct epitem". + * Search the file inside the eventpoll tree. The RB tree operations + * are protected by the "mtx" mutex, and ep_find() must be called with + * "mtx" held. */ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd) { int kcmp; - unsigned long flags; struct rb_node *rbp; struct epitem *epi, *epir = NULL; struct epoll_filefd ffd; ep_set_ffd(&ffd, file, fd); - read_lock_irqsave(&ep->lock, flags); for (rbp = ep->rbr.rb_node; rbp; ) { epi = rb_entry(rbp, struct epitem, rbn); kcmp = ep_cmp_ffd(&ffd, &epi->ffd); @@ -674,12 +594,10 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd) else if (kcmp < 0) rbp = rbp->rb_left; else { - ep_use_epitem(epi); epir = epi; break; } } - read_unlock_irqrestore(&ep->lock, flags); DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_find(%p) -> %p\n", current, file, epir)); @@ -702,7 +620,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k DNPRINTK(3, (KERN_INFO "[%p] eventpoll: poll_callback(%p) epi=%p ep=%p\n", current, epi->ffd.file, epi, ep)); - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); /* * If the event mask does not contain any poll(2) event, we consider the @@ -745,7 +663,7 @@ is_linked: pwake++; out_unlock: - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); /* We have to call this outside the lock */ if (pwake) @@ -796,6 +714,9 @@ static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi) rb_insert_color(&epi->rbn, &ep->rbr); } +/* + * Must be called with "mtx" held. + */ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, struct file *tfile, int fd) { @@ -816,7 +737,6 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, epi->ep = ep; ep_set_ffd(&epi->ffd, tfile, fd); epi->event = *event; - atomic_set(&epi->usecnt, 1); epi->nwait = 0; epi->next = EP_UNACTIVE_PTR; @@ -827,7 +747,9 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, /* * Attach the item to the poll hooks and get current event bits. * We can safely use the file* here because its usage count has - * been increased by the caller of this function. + * been increased by the caller of this function. Note that after + * this operation completes, the poll callback can start hitting + * the new item. */ revents = tfile->f_op->poll(tfile, &epq.pt); @@ -844,12 +766,15 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, list_add_tail(&epi->fllink, &tfile->f_ep_links); spin_unlock(&tfile->f_ep_lock); - /* We have to drop the new item inside our item list to keep track of it */ - write_lock_irqsave(&ep->lock, flags); - - /* Add the current item to the rb-tree */ + /* + * Add the current item to the RB tree. All RB tree operations are + * protected by "mtx", and ep_insert() is called with "mtx" held. + */ ep_rbtree_insert(ep, epi); + /* We have to drop the new item inside our item list to keep track of it */ + spin_lock_irqsave(&ep->lock, flags); + /* If the file is already "ready" we drop it inside the ready list */ if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) { list_add_tail(&epi->rdllink, &ep->rdllist); @@ -861,7 +786,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, pwake++; } - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); /* We have to call this outside the lock */ if (pwake) @@ -879,10 +804,10 @@ error_unregister: * We need to do this because an event could have been arrived on some * allocated wait queue. */ - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); if (ep_is_linked(&epi->rdllink)) list_del_init(&epi->rdllink); - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); kmem_cache_free(epi_cache, epi); error_return: @@ -891,7 +816,7 @@ error_return: /* * Modify the interest event mask by dropping an event if the new mask - * has a match in the current file status. + * has a match in the current file status. Must be called with "mtx" held. */ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event) { @@ -913,36 +838,29 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even */ revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL); - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); /* Copy the data member from inside the lock */ epi->event.data = event->data; /* - * If the item is not linked to the RB tree it means that it's on its - * way toward the removal. Do nothing in this case. + * If the item is "hot" and it is not registered inside the ready + * list, push it inside. If the item is not "hot" and it is currently + * registered inside the ready list, unlink it. */ - if (ep_rb_linked(&epi->rbn)) { - /* - * If the item is "hot" and it is not registered inside the ready - * list, push it inside. If the item is not "hot" and it is currently - * registered inside the ready list, unlink it. - */ - if (revents & event->events) { - if (!ep_is_linked(&epi->rdllink)) { - list_add_tail(&epi->rdllink, &ep->rdllist); + if (revents & event->events) { + if (!ep_is_linked(&epi->rdllink)) { + list_add_tail(&epi->rdllink, &ep->rdllist); - /* Notify waiting tasks that events are available */ - if (waitqueue_active(&ep->wq)) - __wake_up_locked(&ep->wq, TASK_UNINTERRUPTIBLE | - TASK_INTERRUPTIBLE); - if (waitqueue_active(&ep->poll_wait)) - pwake++; - } + /* Notify waiting tasks that events are available */ + if (waitqueue_active(&ep->wq)) + __wake_up_locked(&ep->wq, TASK_UNINTERRUPTIBLE | + TASK_INTERRUPTIBLE); + if (waitqueue_active(&ep->poll_wait)) + pwake++; } } - - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); /* We have to call this outside the lock */ if (pwake) @@ -975,11 +893,11 @@ static int ep_send_events(struct eventpoll *ep, struct epoll_event __user *event * have the poll callback to queue directly on ep->rdllist, * because we are doing it in the loop below, in a lockless way. */ - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); list_splice(&ep->rdllist, &txlist); INIT_LIST_HEAD(&ep->rdllist); ep->ovflist = NULL; - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); /* * We can loop without lock because this is a task private list. @@ -1028,7 +946,7 @@ static int ep_send_events(struct eventpoll *ep, struct epoll_event __user *event errxit: - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); /* * During the time we spent in the loop above, some other events * might have been queued by the poll callback. We re-insert them @@ -1064,7 +982,7 @@ errxit: if (waitqueue_active(&ep->poll_wait)) pwake++; } - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); mutex_unlock(&ep->mtx); @@ -1092,7 +1010,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, MAX_SCHEDULE_TIMEOUT : (timeout * HZ + 999) / 1000; retry: - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); res = 0; if (list_empty(&ep->rdllist)) { @@ -1119,9 +1037,9 @@ retry: break; } - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); jtimeout = schedule_timeout(jtimeout); - write_lock_irqsave(&ep->lock, flags); + spin_lock_irqsave(&ep->lock, flags); } __remove_wait_queue(&ep->wq, &wait); @@ -1131,7 +1049,7 @@ retry: /* Is it worth to try to dig for events ? */ eavail = !list_empty(&ep->rdllist); - write_unlock_irqrestore(&ep->lock, flags); + spin_unlock_irqrestore(&ep->lock, flags); /* * Try to transfer events to user space. In case we get 0 events and @@ -1276,12 +1194,6 @@ asmlinkage long sys_epoll_ctl(int epfd, int op, int fd, error = -ENOENT; break; } - /* - * The function ep_find() increments the usage count of the structure - * so, if this is not NULL, we need to release it. - */ - if (epi) - ep_release_epitem(epi); mutex_unlock(&ep->mtx); error_tgt_fput: @@ -1388,7 +1300,7 @@ asmlinkage long sys_epoll_pwait(int epfd, struct epoll_event __user *events, if (sigmask) { if (error == -EINTR) { memcpy(¤t->saved_sigmask, &sigsaved, - sizeof(sigsaved)); + sizeof(sigsaved)); set_thread_flag(TIF_RESTORE_SIGMASK); } else sigprocmask(SIG_SETMASK, &sigsaved, NULL);