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Commit d47de16c authored by Davide Libenzi's avatar Davide Libenzi Committed by Linus Torvalds
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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: default avatarDavide Libenzi <davidel@xmailserver.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent faa8b6c3
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Inline Side-by-side
Showing with 166 additions and 156 deletions
fs/eventpoll.c
View file @ d47de16c
......@@ -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 @@ struct epoll_filefd {
* 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 {
......@@ -129,6 +129,48 @@ struct poll_safewake {
spinlock_t lock;
};
/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
*/
struct epitem {
/* RB-Tree node used to link this structure to the eventpoll rb-tree */
struct rb_node rbn;
/* List header used to link this structure to the eventpoll ready list */
struct list_head rdllink;
/* The file descriptor information this item refers to */
struct epoll_filefd ffd;
/* Number of active wait queue attached to poll operations */
int nwait;
/* List containing poll wait queues */
struct list_head pwqlist;
/* 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;
};
/*
* This structure is stored inside the "private_data" member of the file
* structure and rapresent the main data sructure for the eventpoll
......@@ -139,12 +181,12 @@ struct eventpoll {
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.
* 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 rw_semaphore sem;
struct mutex mtx;
/* Wait queue used by sys_epoll_wait() */
wait_queue_head_t wq;
......@@ -157,6 +199,13 @@ struct eventpoll {
/* 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 */
......@@ -177,42 +226,6 @@ struct eppoll_entry {
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.
*/
struct epitem {
/* RB-Tree node used to link this structure to the eventpoll rb-tree */
struct rb_node rbn;
/* List header used to link this structure to the eventpoll ready list */
struct list_head rdllink;
/* The file descriptor information this item refers to */
struct epoll_filefd ffd;
/* Number of active wait queue attached to poll operations */
int nwait;
/* List containing poll wait queues */
struct list_head pwqlist;
/* 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;
};
/* Wrapper struct used by poll queueing */
struct ep_pqueue {
poll_table pt;
......@@ -220,7 +233,7 @@ struct ep_pqueue {
};
/*
* 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 @@ static void ep_free(struct eventpoll *ep)
/*
* 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 @@ static void ep_free(struct eventpoll *ep)
/*
* 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 @@ static void ep_free(struct eventpoll *ep)
}
mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
}
static int ep_eventpoll_release(struct inode *inode, struct file *file)
......@@ -594,9 +609,9 @@ void eventpoll_release_file(struct file *file)
* 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 @@ void eventpoll_release_file(struct file *file)
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 @@ static int ep_alloc(struct eventpoll **pep)
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 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
* 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 @@ is_linked:
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 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
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 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
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(&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);
INIT_LIST_HEAD(&injlist);
/*
* 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 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
/*
* 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 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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().
*/
list_del(&epi->rdllink);
* 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.
*/
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);
}
list_add_tail(&epi->rdllink, &ep->rdllist);
}
error = 0;
errxit:
errxit:
write_lock_irqsave(&ep->lock, flags);
/*
* If the re-injection list or the txlist are not empty, re-splice
* them to the ready list and do proper wakeups.
* 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.
*/
if (!list_empty(&injlist) || !list_empty(txlist)) {
write_lock_irqsave(&ep->lock, flags);
ep->ovflist = EP_UNACTIVE_PTR;
list_splice(txlist, &ep->rdllist);
list_splice(&injlist, &ep->rdllist);
/*
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
* 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.
*/
list_splice(&txlist, &ep->rdllist);
if (!list_empty(&ep->rdllist)) {
/*
* Wake up (if active) both the eventpoll wait list and the ->poll()
* wait list.
*/
if (waitqueue_active(&ep->wq))
......@@ -1020,9 +1063,10 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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 @@ retry:
* 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 @@ retry:
* 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 @@ asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
*/
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 @@ asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
*/
if (epi)
ep_release_epitem(epi);
up_write(&ep->sem);
mutex_unlock(&ep->mtx);
error_tgt_fput:
fput(tfile);
......
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