fix epoll single pass code and add wait-exclusive flag
Fixes the epoll single pass code. During the unlocked event delivery (to
userspace) code, the poll callback can re-issue new events, and we must
receive them correctly. Since we loop in a lockless fashion, we want to be
O(nready), and we don't want to flash on/off the spinlock for every event, we
have the poll callback to use a secondary list to queue events while we're
inside the event delivery loop. The rw_semaphore has been turned into a
mutex. This patch also adds the wait-exclusive flag, as suggested by Davi
Arnaut.
Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1aad34e..1dbedc7 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -26,7 +26,6 @@
#include <linux/hash.h>
#include <linux/spinlock.h>
#include <linux/syscalls.h>
-#include <linux/rwsem.h>
#include <linux/rbtree.h>
#include <linux/wait.h>
#include <linux/eventpoll.h>
@@ -39,14 +38,13 @@
#include <asm/io.h>
#include <asm/mman.h>
#include <asm/atomic.h>
-#include <asm/semaphore.h>
/*
* LOCKING:
* There are three level of locking required by epoll :
*
* 1) epmutex (mutex)
- * 2) ep->sem (rw_semaphore)
+ * 2) ep->mtx (mutes)
* 3) ep->lock (rw_lock)
*
* The acquire order is the one listed above, from 1 to 3.
@@ -57,20 +55,20 @@
* a spinlock. During the event transfer loop (from kernel to
* user space) we could end up sleeping due a copy_to_user(), so
* we need a lock that will allow us to sleep. This lock is a
- * read-write semaphore (ep->sem). It is acquired on read during
- * the event transfer loop and in write during epoll_ctl(EPOLL_CTL_DEL)
- * and during eventpoll_release_file(). Then we also need a global
- * semaphore to serialize eventpoll_release_file() and ep_free().
- * This semaphore is acquired by ep_free() during the epoll file
+ * mutex (ep->mtx). It is acquired during the event transfer loop,
+ * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
+ * Then we also need a global mutex to serialize eventpoll_release_file()
+ * and ep_free().
+ * This mutex is acquired by ep_free() during the epoll file
* cleanup path and it is also acquired by eventpoll_release_file()
* if a file has been pushed inside an epoll set and it is then
* close()d without a previous call toepoll_ctl(EPOLL_CTL_DEL).
- * It is possible to drop the "ep->sem" and to use the global
- * semaphore "epmutex" (together with "ep->lock") to have it working,
- * but having "ep->sem" will make the interface more scalable.
+ * It is possible to drop the "ep->mtx" and to use the global
+ * mutex "epmutex" (together with "ep->lock") to have it working,
+ * but having "ep->mtx" will make the interface more scalable.
* Events that require holding "epmutex" are very rare, while for
- * normal operations the epoll private "ep->sem" will guarantee
- * a greater scalability.
+ * normal operations the epoll private "ep->mtx" will guarantee
+ * a better scalability.
*/
#define DEBUG_EPOLL 0
@@ -102,6 +100,8 @@
#define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
+#define EP_UNACTIVE_PTR ((void *) -1L)
+
struct epoll_filefd {
struct file *file;
int fd;
@@ -111,7 +111,7 @@
* Node that is linked into the "wake_task_list" member of the "struct poll_safewake".
* It is used to keep track on all tasks that are currently inside the wake_up() code
* to 1) short-circuit the one coming from the same task and same wait queue head
- * ( loop ) 2) allow a maximum number of epoll descriptors inclusion nesting
+ * (loop) 2) allow a maximum number of epoll descriptors inclusion nesting
* 3) let go the ones coming from other tasks.
*/
struct wake_task_node {
@@ -130,54 +130,6 @@
};
/*
- * This structure is stored inside the "private_data" member of the file
- * structure and rapresent the main data sructure for the eventpoll
- * interface.
- */
-struct eventpoll {
- /* Protect the this structure access */
- rwlock_t lock;
-
- /*
- * This semaphore is used to ensure that files are not removed
- * while epoll is using them. This is read-held during the event
- * collection loop and it is write-held during the file cleanup
- * path, the epoll file exit code and the ctl operations.
- */
- struct rw_semaphore sem;
-
- /* Wait queue used by sys_epoll_wait() */
- wait_queue_head_t wq;
-
- /* Wait queue used by file->poll() */
- wait_queue_head_t poll_wait;
-
- /* List of ready file descriptors */
- struct list_head rdllist;
-
- /* RB-Tree root used to store monitored fd structs */
- struct rb_root rbr;
-};
-
-/* Wait structure used by the poll hooks */
-struct eppoll_entry {
- /* List header used to link this structure to the "struct epitem" */
- struct list_head llink;
-
- /* The "base" pointer is set to the container "struct epitem" */
- void *base;
-
- /*
- * Wait queue item that will be linked to the target file wait
- * queue head.
- */
- wait_queue_t wait;
-
- /* The wait queue head that linked the "wait" wait queue item */
- wait_queue_head_t *whead;
-};
-
-/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
*/
@@ -211,6 +163,67 @@
/* 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;
+};
+
+/*
+ * This structure is stored inside the "private_data" member of the file
+ * structure and rapresent the main data sructure for the eventpoll
+ * interface.
+ */
+struct eventpoll {
+ /* Protect the this structure access */
+ rwlock_t lock;
+
+ /*
+ * This mutex is used to ensure that files are not removed
+ * while epoll is using them. This is held during the event
+ * collection loop, the file cleanup path, the epoll file exit
+ * code and the ctl operations.
+ */
+ struct mutex mtx;
+
+ /* Wait queue used by sys_epoll_wait() */
+ wait_queue_head_t wq;
+
+ /* Wait queue used by file->poll() */
+ wait_queue_head_t poll_wait;
+
+ /* List of ready file descriptors */
+ struct list_head rdllist;
+
+ /* RB-Tree root used to store monitored fd structs */
+ struct rb_root rbr;
+
+ /*
+ * This is a single linked list that chains all the "struct epitem" that
+ * happened while transfering ready events to userspace w/out
+ * holding ->lock.
+ */
+ struct epitem *ovflist;
+};
+
+/* Wait structure used by the poll hooks */
+struct eppoll_entry {
+ /* List header used to link this structure to the "struct epitem" */
+ struct list_head llink;
+
+ /* The "base" pointer is set to the container "struct epitem" */
+ void *base;
+
+ /*
+ * Wait queue item that will be linked to the target file wait
+ * queue head.
+ */
+ wait_queue_t wait;
+
+ /* The wait queue head that linked the "wait" wait queue item */
+ wait_queue_head_t *whead;
};
/* Wrapper struct used by poll queueing */
@@ -220,7 +233,7 @@
};
/*
- * This semaphore is used to serialize ep_free() and eventpoll_release_file().
+ * This mutex is used to serialize ep_free() and eventpoll_release_file().
*/
static struct mutex epmutex;
@@ -506,7 +519,7 @@
/*
* We need to lock this because we could be hit by
* eventpoll_release_file() while we're freeing the "struct eventpoll".
- * We do not need to hold "ep->sem" here because the epoll file
+ * We do not need to hold "ep->mtx" here because the epoll file
* is on the way to be removed and no one has references to it
* anymore. The only hit might come from eventpoll_release_file() but
* holding "epmutex" is sufficent here.
@@ -525,7 +538,7 @@
/*
* Walks through the whole tree by freeing each "struct epitem". At this
* point we are sure no poll callbacks will be lingering around, and also by
- * write-holding "sem" we can be sure that no file cleanup code will hit
+ * holding "epmutex" we can be sure that no file cleanup code will hit
* us during this operation. So we can avoid the lock on "ep->lock".
*/
while ((rbp = rb_first(&ep->rbr)) != 0) {
@@ -534,6 +547,8 @@
}
mutex_unlock(&epmutex);
+
+ mutex_destroy(&ep->mtx);
}
static int ep_eventpoll_release(struct inode *inode, struct file *file)
@@ -594,9 +609,9 @@
* We don't want to get "file->f_ep_lock" because it is not
* necessary. It is not necessary because we're in the "struct file"
* cleanup path, and this means that noone is using this file anymore.
- * The only hit might come from ep_free() but by holding the semaphore
+ * The only hit might come from ep_free() but by holding the mutex
* will correctly serialize the operation. We do need to acquire
- * "ep->sem" after "epmutex" because ep_remove() requires it when called
+ * "ep->mtx" after "epmutex" because ep_remove() requires it when called
* from anywhere but ep_free().
*/
mutex_lock(&epmutex);
@@ -606,9 +621,9 @@
ep = epi->ep;
list_del_init(&epi->fllink);
- down_write(&ep->sem);
+ mutex_lock(&ep->mtx);
ep_remove(ep, epi);
- up_write(&ep->sem);
+ mutex_unlock(&ep->mtx);
}
mutex_unlock(&epmutex);
@@ -622,11 +637,12 @@
return -ENOMEM;
rwlock_init(&ep->lock);
- init_rwsem(&ep->sem);
+ mutex_init(&ep->mtx);
init_waitqueue_head(&ep->wq);
init_waitqueue_head(&ep->poll_wait);
INIT_LIST_HEAD(&ep->rdllist);
ep->rbr = RB_ROOT;
+ ep->ovflist = EP_UNACTIVE_PTR;
*pep = ep;
@@ -695,7 +711,21 @@
* until the next EPOLL_CTL_MOD will be issued.
*/
if (!(epi->event.events & ~EP_PRIVATE_BITS))
- goto is_disabled;
+ goto out_unlock;
+
+ /*
+ * If we are trasfering events to userspace, we can hold no locks
+ * (because we're accessing user memory, and because of linux f_op->poll()
+ * semantics). All the events that happens during that period of time are
+ * chained in ep->ovflist and requeued later on.
+ */
+ if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
+ if (epi->next == EP_UNACTIVE_PTR) {
+ epi->next = ep->ovflist;
+ ep->ovflist = epi;
+ }
+ goto out_unlock;
+ }
/* If this file is already in the ready list we exit soon */
if (ep_is_linked(&epi->rdllink))
@@ -714,7 +744,7 @@
if (waitqueue_active(&ep->poll_wait))
pwake++;
-is_disabled:
+out_unlock:
write_unlock_irqrestore(&ep->lock, flags);
/* We have to call this outside the lock */
@@ -788,6 +818,7 @@
epi->event = *event;
atomic_set(&epi->usecnt, 1);
epi->nwait = 0;
+ epi->next = EP_UNACTIVE_PTR;
/* Initialize the poll table using the queue callback */
epq.epi = epi;
@@ -920,36 +951,50 @@
return 0;
}
-/*
- * This function is called without holding the "ep->lock" since the call to
- * __copy_to_user() might sleep, and also f_op->poll() might reenable the IRQ
- * because of the way poll() is traditionally implemented in Linux.
- */
-static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
- struct epoll_event __user *events, int maxevents)
+static int ep_send_events(struct eventpoll *ep, struct epoll_event __user *events,
+ int maxevents)
{
int eventcnt, error = -EFAULT, pwake = 0;
unsigned int revents;
unsigned long flags;
- struct epitem *epi;
- struct list_head injlist;
+ struct epitem *epi, *nepi;
+ struct list_head txlist;
- INIT_LIST_HEAD(&injlist);
+ INIT_LIST_HEAD(&txlist);
+
+ /*
+ * We need to lock this because we could be hit by
+ * eventpoll_release_file() and epoll_ctl(EPOLL_CTL_DEL).
+ */
+ mutex_lock(&ep->mtx);
+
+ /*
+ * Steal the ready list, and re-init the original one to the
+ * empty list. Also, set ep->ovflist to NULL so that events
+ * happening while looping w/out locks, are not lost. We cannot
+ * 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);
+ list_splice(&ep->rdllist, &txlist);
+ INIT_LIST_HEAD(&ep->rdllist);
+ ep->ovflist = NULL;
+ write_unlock_irqrestore(&ep->lock, flags);
/*
* We can loop without lock because this is a task private list.
* We just splice'd out the ep->rdllist in ep_collect_ready_items().
- * Items cannot vanish during the loop because we are holding "sem" in
- * read.
+ * Items cannot vanish during the loop because we are holding "mtx".
*/
- for (eventcnt = 0; !list_empty(txlist) && eventcnt < maxevents;) {
- epi = list_first_entry(txlist, struct epitem, rdllink);
- prefetch(epi->rdllink.next);
+ for (eventcnt = 0; !list_empty(&txlist) && eventcnt < maxevents;) {
+ epi = list_first_entry(&txlist, struct epitem, rdllink);
+
+ list_del_init(&epi->rdllink);
/*
* Get the ready file event set. We can safely use the file
- * because we are holding the "sem" in read and this will
- * guarantee that both the file and the item will not vanish.
+ * because we are holding the "mtx" and this will guarantee
+ * that both the file and the item will not vanish.
*/
revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
revents &= epi->event.events;
@@ -957,8 +1002,8 @@
/*
* Is the event mask intersect the caller-requested one,
* deliver the event to userspace. Again, we are holding
- * "sem" in read, so no operations coming from userspace
- * can change the item.
+ * "mtx", so no operations coming from userspace can change
+ * the item.
*/
if (revents) {
if (__put_user(revents,
@@ -970,49 +1015,47 @@
epi->event.events &= EP_PRIVATE_BITS;
eventcnt++;
}
-
/*
- * This is tricky. We are holding the "sem" in read, and this
- * means that the operations that can change the "linked" status
- * of the epoll item (epi->rbn and epi->rdllink), cannot touch
- * them. Also, since we are "linked" from a epi->rdllink POV
- * (the item is linked to our transmission list we just
- * spliced), the ep_poll_callback() cannot touch us either,
- * because of the check present in there. Another parallel
- * epoll_wait() will not get the same result set, since we
- * spliced the ready list before. Note that list_del() still
- * shows the item as linked to the test in ep_poll_callback().
+ * At this point, noone can insert into ep->rdllist besides
+ * us. The epoll_ctl() callers are locked out by us holding
+ * "mtx" and the poll callback will queue them in ep->ovflist.
*/
- list_del(&epi->rdllink);
if (!(epi->event.events & EPOLLET) &&
- (revents & epi->event.events))
- list_add_tail(&epi->rdllink, &injlist);
- else {
- /*
- * Be sure the item is totally detached before re-init
- * the list_head. After INIT_LIST_HEAD() is committed,
- * the ep_poll_callback() can requeue the item again,
- * but we don't care since we are already past it.
- */
- smp_mb();
- INIT_LIST_HEAD(&epi->rdllink);
- }
+ (revents & epi->event.events))
+ list_add_tail(&epi->rdllink, &ep->rdllist);
}
error = 0;
- errxit:
+errxit:
+
+ write_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
+ * here (in case they are not already queued, or they're one-shot).
+ */
+ for (nepi = ep->ovflist; (epi = nepi) != NULL;
+ nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
+ if (!ep_is_linked(&epi->rdllink) &&
+ (epi->event.events & ~EP_PRIVATE_BITS))
+ list_add_tail(&epi->rdllink, &ep->rdllist);
+ }
+ /*
+ * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
+ * releasing the lock, events will be queued in the normal way inside
+ * ep->rdllist.
+ */
+ ep->ovflist = EP_UNACTIVE_PTR;
/*
- * If the re-injection list or the txlist are not empty, re-splice
- * them to the ready list and do proper wakeups.
+ * In case of error in the event-send loop, we might still have items
+ * inside the "txlist". We need to splice them back inside ep->rdllist.
*/
- if (!list_empty(&injlist) || !list_empty(txlist)) {
- write_lock_irqsave(&ep->lock, flags);
+ list_splice(&txlist, &ep->rdllist);
- list_splice(txlist, &ep->rdllist);
- list_splice(&injlist, &ep->rdllist);
+ if (!list_empty(&ep->rdllist)) {
/*
- * Wake up ( if active ) both the eventpoll wait list and the ->poll()
+ * Wake up (if active) both the eventpoll wait list and the ->poll()
* wait list.
*/
if (waitqueue_active(&ep->wq))
@@ -1020,9 +1063,10 @@
TASK_INTERRUPTIBLE);
if (waitqueue_active(&ep->poll_wait))
pwake++;
-
- write_unlock_irqrestore(&ep->lock, flags);
}
+ write_unlock_irqrestore(&ep->lock, flags);
+
+ mutex_unlock(&ep->mtx);
/* We have to call this outside the lock */
if (pwake)
@@ -1031,41 +1075,6 @@
return eventcnt == 0 ? error: eventcnt;
}
-/*
- * Perform the transfer of events to user space.
- */
-static int ep_events_transfer(struct eventpoll *ep,
- struct epoll_event __user *events, int maxevents)
-{
- int eventcnt;
- unsigned long flags;
- struct list_head txlist;
-
- INIT_LIST_HEAD(&txlist);
-
- /*
- * We need to lock this because we could be hit by
- * eventpoll_release_file() and epoll_ctl(EPOLL_CTL_DEL).
- */
- down_read(&ep->sem);
-
- /*
- * Steal the ready list, and re-init the original one to the
- * empty list.
- */
- write_lock_irqsave(&ep->lock, flags);
- list_splice(&ep->rdllist, &txlist);
- INIT_LIST_HEAD(&ep->rdllist);
- write_unlock_irqrestore(&ep->lock, flags);
-
- /* Build result set in userspace */
- eventcnt = ep_send_events(ep, &txlist, events, maxevents);
-
- up_read(&ep->sem);
-
- return eventcnt;
-}
-
static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
int maxevents, long timeout)
{
@@ -1093,6 +1102,7 @@
* ep_poll_callback() when events will become available.
*/
init_waitqueue_entry(&wait, current);
+ wait.flags |= WQ_FLAG_EXCLUSIVE;
__add_wait_queue(&ep->wq, &wait);
for (;;) {
@@ -1129,7 +1139,7 @@
* more luck.
*/
if (!res && eavail &&
- !(res = ep_events_transfer(ep, events, maxevents)) && jtimeout)
+ !(res = ep_send_events(ep, events, maxevents)) && jtimeout)
goto retry;
return res;
@@ -1237,7 +1247,7 @@
*/
ep = file->private_data;
- down_write(&ep->sem);
+ mutex_lock(&ep->mtx);
/* Try to lookup the file inside our RB tree */
epi = ep_find(ep, tfile, fd);
@@ -1272,7 +1282,7 @@
*/
if (epi)
ep_release_epitem(epi);
- up_write(&ep->sem);
+ mutex_unlock(&ep->mtx);
error_tgt_fput:
fput(tfile);