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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: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Davide Libenzi 2007-05-15 01:40:41 -07:00 committed by Linus Torvalds
parent faa8b6c3c2
commit d47de16c72
1 changed files with 173 additions and 163 deletions
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336
fs/eventpoll.c
View File

@ -26,7 +26,6 @@
#include <linux/hash.h> #include <linux/hash.h>
#include <linux/spinlock.h> #include <linux/spinlock.h>
#include <linux/syscalls.h> #include <linux/syscalls.h>
#include <linux/rwsem.h>
#include <linux/rbtree.h> #include <linux/rbtree.h>
#include <linux/wait.h> #include <linux/wait.h>
#include <linux/eventpoll.h> #include <linux/eventpoll.h>
@ -39,14 +38,13 @@
#include <asm/io.h> #include <asm/io.h>
#include <asm/mman.h> #include <asm/mman.h>
#include <asm/atomic.h> #include <asm/atomic.h>
#include <asm/semaphore.h>
/* /*
* LOCKING: * LOCKING:
* There are three level of locking required by epoll : * There are three level of locking required by epoll :
* *
* 1) epmutex (mutex) * 1) epmutex (mutex)
* 2) ep->sem (rw_semaphore) * 2) ep->mtx (mutes)
* 3) ep->lock (rw_lock) * 3) ep->lock (rw_lock)
* *
* The acquire order is the one listed above, from 1 to 3. * 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 * a spinlock. During the event transfer loop (from kernel to
* user space) we could end up sleeping due a copy_to_user(), so * 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 * 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 * mutex (ep->mtx). It is acquired during the event transfer loop,
* the event transfer loop and in write during epoll_ctl(EPOLL_CTL_DEL) * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
* and during eventpoll_release_file(). Then we also need a global * Then we also need a global mutex to serialize eventpoll_release_file()
* semaphore to serialize eventpoll_release_file() and ep_free(). * and ep_free().
* This semaphore is acquired by ep_free() during the epoll file * This mutex is acquired by ep_free() during the epoll file
* cleanup path and it is also acquired by eventpoll_release_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 * 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). * close()d without a previous call toepoll_ctl(EPOLL_CTL_DEL).
* It is possible to drop the "ep->sem" and to use the global * It is possible to drop the "ep->mtx" and to use the global
* semaphore "epmutex" (together with "ep->lock") to have it working, * mutex "epmutex" (together with "ep->lock") to have it working,
* but having "ep->sem" will make the interface more scalable. * but having "ep->mtx" will make the interface more scalable.
* Events that require holding "epmutex" are very rare, while for * Events that require holding "epmutex" are very rare, while for
* normal operations the epoll private "ep->sem" will guarantee * normal operations the epoll private "ep->mtx" will guarantee
* a greater scalability. * a better scalability.
*/ */
#define DEBUG_EPOLL 0 #define DEBUG_EPOLL 0
@ -102,6 +100,8 @@
#define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event)) #define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
#define EP_UNACTIVE_PTR ((void *) -1L)
struct epoll_filefd { struct epoll_filefd {
struct file *file; struct file *file;
int fd; int fd;
@ -111,7 +111,7 @@ struct epoll_filefd {
* Node that is linked into the "wake_task_list" member of the "struct poll_safewake". * 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 * 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 * 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. * 3) let go the ones coming from other tasks.
*/ */
struct wake_task_node { struct wake_task_node {
@ -129,54 +129,6 @@ struct poll_safewake {
spinlock_t lock; spinlock_t lock;
}; };
/*
* 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 * Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree. * have an entry of this type linked to the "rbr" RB tree.
@ -211,6 +163,67 @@ struct epitem {
/* List header used to link this item to the "struct file" items list */ /* List header used to link this item to the "struct file" items list */
struct list_head fllink; 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 */ /* Wrapper struct used by poll queueing */
@ -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; 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 * We need to lock this because we could be hit by
* eventpoll_release_file() while we're freeing the "struct eventpoll". * 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 * 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 * anymore. The only hit might come from eventpoll_release_file() but
* holding "epmutex" is sufficent here. * 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 * 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 * 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". * us during this operation. So we can avoid the lock on "ep->lock".
*/ */
while ((rbp = rb_first(&ep->rbr)) != 0) { while ((rbp = rb_first(&ep->rbr)) != 0) {
@ -534,6 +547,8 @@ static void ep_free(struct eventpoll *ep)
} }
mutex_unlock(&epmutex); mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
} }
static int ep_eventpoll_release(struct inode *inode, struct file *file) 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 * 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" * necessary. It is not necessary because we're in the "struct file"
* cleanup path, and this means that noone is using this file anymore. * 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 * 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(). * from anywhere but ep_free().
*/ */
mutex_lock(&epmutex); mutex_lock(&epmutex);
@ -606,9 +621,9 @@ void eventpoll_release_file(struct file *file)
ep = epi->ep; ep = epi->ep;
list_del_init(&epi->fllink); list_del_init(&epi->fllink);
down_write(&ep->sem); mutex_lock(&ep->mtx);
ep_remove(ep, epi); ep_remove(ep, epi);
up_write(&ep->sem); mutex_unlock(&ep->mtx);
} }
mutex_unlock(&epmutex); mutex_unlock(&epmutex);
@ -622,11 +637,12 @@ static int ep_alloc(struct eventpoll **pep)
return -ENOMEM; return -ENOMEM;
rwlock_init(&ep->lock); rwlock_init(&ep->lock);
init_rwsem(&ep->sem); mutex_init(&ep->mtx);
init_waitqueue_head(&ep->wq); init_waitqueue_head(&ep->wq);
init_waitqueue_head(&ep->poll_wait); init_waitqueue_head(&ep->poll_wait);
INIT_LIST_HEAD(&ep->rdllist); INIT_LIST_HEAD(&ep->rdllist);
ep->rbr = RB_ROOT; ep->rbr = RB_ROOT;
ep->ovflist = EP_UNACTIVE_PTR;
*pep = ep; *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. * until the next EPOLL_CTL_MOD will be issued.
*/ */
if (!(epi->event.events & ~EP_PRIVATE_BITS)) 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 this file is already in the ready list we exit soon */
if (ep_is_linked(&epi->rdllink)) if (ep_is_linked(&epi->rdllink))
@ -714,7 +744,7 @@ is_linked:
if (waitqueue_active(&ep->poll_wait)) if (waitqueue_active(&ep->poll_wait))
pwake++; pwake++;
is_disabled: out_unlock:
write_unlock_irqrestore(&ep->lock, flags); write_unlock_irqrestore(&ep->lock, flags);
/* We have to call this outside the lock */ /* 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; epi->event = *event;
atomic_set(&epi->usecnt, 1); atomic_set(&epi->usecnt, 1);
epi->nwait = 0; epi->nwait = 0;
epi->next = EP_UNACTIVE_PTR;
/* Initialize the poll table using the queue callback */ /* Initialize the poll table using the queue callback */
epq.epi = epi; epq.epi = epi;
@ -920,36 +951,50 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
return 0; return 0;
} }
/* static int ep_send_events(struct eventpoll *ep, struct epoll_event __user *events,
* This function is called without holding the "ep->lock" since the call to int maxevents)
* __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)
{ {
int eventcnt, error = -EFAULT, pwake = 0; int eventcnt, error = -EFAULT, pwake = 0;
unsigned int revents; unsigned int revents;
unsigned long flags; unsigned long flags;
struct epitem *epi; struct epitem *epi, *nepi;
struct list_head injlist; 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 can loop without lock because this is a task private list.
* We just splice'd out the ep->rdllist in ep_collect_ready_items(). * 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 * Items cannot vanish during the loop because we are holding "mtx".
* read.
*/ */
for (eventcnt = 0; !list_empty(txlist) && eventcnt < maxevents;) { for (eventcnt = 0; !list_empty(&txlist) && eventcnt < maxevents;) {
epi = list_first_entry(txlist, struct epitem, rdllink); epi = list_first_entry(&txlist, struct epitem, rdllink);
prefetch(epi->rdllink.next);
list_del_init(&epi->rdllink);
/* /*
* Get the ready file event set. We can safely use the file * Get the ready file event set. We can safely use the file
* because we are holding the "sem" in read and this will * because we are holding the "mtx" and this will guarantee
* guarantee that both the file and the item will not vanish. * that both the file and the item will not vanish.
*/ */
revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL); revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
revents &= epi->event.events; 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, * Is the event mask intersect the caller-requested one,
* deliver the event to userspace. Again, we are holding * deliver the event to userspace. Again, we are holding
* "sem" in read, so no operations coming from userspace * "mtx", so no operations coming from userspace can change
* can change the item. * the item.
*/ */
if (revents) { if (revents) {
if (__put_user(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; epi->event.events &= EP_PRIVATE_BITS;
eventcnt++; eventcnt++;
} }
/* /*
* This is tricky. We are holding the "sem" in read, and this * At this point, noone can insert into ep->rdllist besides
* means that the operations that can change the "linked" status * us. The epoll_ctl() callers are locked out by us holding
* of the epoll item (epi->rbn and epi->rdllink), cannot touch * "mtx" and the poll callback will queue them in ep->ovflist.
* 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);
if (!(epi->event.events & EPOLLET) && if (!(epi->event.events & EPOLLET) &&
(revents & epi->event.events)) (revents & epi->event.events))
list_add_tail(&epi->rdllink, &injlist); list_add_tail(&epi->rdllink, &ep->rdllist);
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);
}
} }
error = 0; 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 * In case of error in the event-send loop, we might still have items
* them to the ready list and do proper wakeups. * inside the "txlist". We need to splice them back inside ep->rdllist.
*/ */
if (!list_empty(&injlist) || !list_empty(txlist)) { list_splice(&txlist, &ep->rdllist);
write_lock_irqsave(&ep->lock, flags);
list_splice(txlist, &ep->rdllist); if (!list_empty(&ep->rdllist)) {
list_splice(&injlist, &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. * wait list.
*/ */
if (waitqueue_active(&ep->wq)) if (waitqueue_active(&ep->wq))
@ -1020,9 +1063,10 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
TASK_INTERRUPTIBLE); TASK_INTERRUPTIBLE);
if (waitqueue_active(&ep->poll_wait)) if (waitqueue_active(&ep->poll_wait))
pwake++; 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 */ /* We have to call this outside the lock */
if (pwake) if (pwake)
@ -1031,41 +1075,6 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
return eventcnt == 0 ? error: eventcnt; 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, static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
int maxevents, long timeout) int maxevents, long timeout)
{ {
@ -1093,6 +1102,7 @@ retry:
* ep_poll_callback() when events will become available. * ep_poll_callback() when events will become available.
*/ */
init_waitqueue_entry(&wait, current); init_waitqueue_entry(&wait, current);
wait.flags |= WQ_FLAG_EXCLUSIVE;
__add_wait_queue(&ep->wq, &wait); __add_wait_queue(&ep->wq, &wait);
for (;;) { for (;;) {
@ -1129,7 +1139,7 @@ retry:
* more luck. * more luck.
*/ */
if (!res && eavail && if (!res && eavail &&
!(res = ep_events_transfer(ep, events, maxevents)) && jtimeout) !(res = ep_send_events(ep, events, maxevents)) && jtimeout)
goto retry; goto retry;
return res; return res;
@ -1237,7 +1247,7 @@ asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
*/ */
ep = file->private_data; ep = file->private_data;
down_write(&ep->sem); mutex_lock(&ep->mtx);
/* Try to lookup the file inside our RB tree */ /* Try to lookup the file inside our RB tree */
epi = ep_find(ep, tfile, fd); epi = ep_find(ep, tfile, fd);
@ -1272,7 +1282,7 @@ asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
*/ */
if (epi) if (epi)
ep_release_epitem(epi); ep_release_epitem(epi);
up_write(&ep->sem); mutex_unlock(&ep->mtx);
error_tgt_fput: error_tgt_fput:
fput(tfile); fput(tfile);