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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
Linus Torvalds
parent
faa8b6c3c2
commit
d47de16c72
1 changed files with 173 additions and 163 deletions
336
fs/eventpoll.c
336
fs/eventpoll.c
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@ -26,7 +26,6 @@
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#include <linux/hash.h>
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#include <linux/spinlock.h>
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#include <linux/syscalls.h>
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#include <linux/rwsem.h>
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#include <linux/rbtree.h>
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#include <linux/wait.h>
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#include <linux/eventpoll.h>
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@ -39,14 +38,13 @@
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#include <asm/io.h>
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#include <asm/mman.h>
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#include <asm/atomic.h>
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#include <asm/semaphore.h>
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/*
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* LOCKING:
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* There are three level of locking required by epoll :
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*
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* 1) epmutex (mutex)
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* 2) ep->sem (rw_semaphore)
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* 2) ep->mtx (mutes)
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* 3) ep->lock (rw_lock)
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*
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* The acquire order is the one listed above, from 1 to 3.
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@ -57,20 +55,20 @@
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* a spinlock. During the event transfer loop (from kernel to
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* user space) we could end up sleeping due a copy_to_user(), so
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* we need a lock that will allow us to sleep. This lock is a
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* read-write semaphore (ep->sem). It is acquired on read during
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* the event transfer loop and in write during epoll_ctl(EPOLL_CTL_DEL)
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* and during eventpoll_release_file(). Then we also need a global
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* semaphore to serialize eventpoll_release_file() and ep_free().
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* This semaphore is acquired by ep_free() during the epoll file
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* mutex (ep->mtx). It is acquired during the event transfer loop,
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* during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
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* Then we also need a global mutex to serialize eventpoll_release_file()
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* and ep_free().
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* This mutex is acquired by ep_free() during the epoll file
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* cleanup path and it is also acquired by eventpoll_release_file()
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* if a file has been pushed inside an epoll set and it is then
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* close()d without a previous call toepoll_ctl(EPOLL_CTL_DEL).
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* It is possible to drop the "ep->sem" and to use the global
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* semaphore "epmutex" (together with "ep->lock") to have it working,
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* but having "ep->sem" will make the interface more scalable.
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* It is possible to drop the "ep->mtx" and to use the global
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* mutex "epmutex" (together with "ep->lock") to have it working,
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* but having "ep->mtx" will make the interface more scalable.
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* Events that require holding "epmutex" are very rare, while for
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* normal operations the epoll private "ep->sem" will guarantee
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* a greater scalability.
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* normal operations the epoll private "ep->mtx" will guarantee
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* a better scalability.
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*/
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#define DEBUG_EPOLL 0
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@ -102,6 +100,8 @@
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#define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
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#define EP_UNACTIVE_PTR ((void *) -1L)
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struct epoll_filefd {
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struct file *file;
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int fd;
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@ -111,7 +111,7 @@ struct epoll_filefd {
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* Node that is linked into the "wake_task_list" member of the "struct poll_safewake".
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* It is used to keep track on all tasks that are currently inside the wake_up() code
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* to 1) short-circuit the one coming from the same task and same wait queue head
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* ( loop ) 2) allow a maximum number of epoll descriptors inclusion nesting
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* (loop) 2) allow a maximum number of epoll descriptors inclusion nesting
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* 3) let go the ones coming from other tasks.
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*/
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struct wake_task_node {
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@ -129,54 +129,6 @@ struct poll_safewake {
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spinlock_t lock;
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};
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/*
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* This structure is stored inside the "private_data" member of the file
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* structure and rapresent the main data sructure for the eventpoll
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* interface.
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*/
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struct eventpoll {
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/* Protect the this structure access */
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rwlock_t lock;
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/*
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* This semaphore is used to ensure that files are not removed
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* while epoll is using them. This is read-held during the event
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* collection loop and it is write-held during the file cleanup
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* path, the epoll file exit code and the ctl operations.
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*/
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struct rw_semaphore sem;
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/* Wait queue used by sys_epoll_wait() */
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wait_queue_head_t wq;
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/* Wait queue used by file->poll() */
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wait_queue_head_t poll_wait;
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/* List of ready file descriptors */
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struct list_head rdllist;
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/* RB-Tree root used to store monitored fd structs */
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struct rb_root rbr;
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};
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/* Wait structure used by the poll hooks */
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struct eppoll_entry {
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/* List header used to link this structure to the "struct epitem" */
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struct list_head llink;
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/* The "base" pointer is set to the container "struct epitem" */
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void *base;
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/*
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* Wait queue item that will be linked to the target file wait
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* queue head.
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*/
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wait_queue_t wait;
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/* The wait queue head that linked the "wait" wait queue item */
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wait_queue_head_t *whead;
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};
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/*
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* Each file descriptor added to the eventpoll interface will
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* have an entry of this type linked to the "rbr" RB tree.
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@ -211,6 +163,67 @@ struct epitem {
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/* List header used to link this item to the "struct file" items list */
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struct list_head fllink;
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/*
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* Works together "struct eventpoll"->ovflist in keeping the
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* single linked chain of items.
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*/
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struct epitem *next;
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};
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/*
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* This structure is stored inside the "private_data" member of the file
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* structure and rapresent the main data sructure for the eventpoll
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* interface.
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*/
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struct eventpoll {
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/* Protect the this structure access */
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rwlock_t lock;
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/*
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* This mutex is used to ensure that files are not removed
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* while epoll is using them. This is held during the event
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* collection loop, the file cleanup path, the epoll file exit
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* code and the ctl operations.
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*/
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struct mutex mtx;
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/* Wait queue used by sys_epoll_wait() */
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wait_queue_head_t wq;
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/* Wait queue used by file->poll() */
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wait_queue_head_t poll_wait;
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/* List of ready file descriptors */
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struct list_head rdllist;
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/* RB-Tree root used to store monitored fd structs */
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struct rb_root rbr;
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/*
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* This is a single linked list that chains all the "struct epitem" that
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* happened while transfering ready events to userspace w/out
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* holding ->lock.
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*/
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struct epitem *ovflist;
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};
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/* Wait structure used by the poll hooks */
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struct eppoll_entry {
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/* List header used to link this structure to the "struct epitem" */
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struct list_head llink;
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/* The "base" pointer is set to the container "struct epitem" */
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void *base;
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/*
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* Wait queue item that will be linked to the target file wait
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* queue head.
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*/
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wait_queue_t wait;
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/* The wait queue head that linked the "wait" wait queue item */
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wait_queue_head_t *whead;
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};
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/* Wrapper struct used by poll queueing */
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@ -220,7 +233,7 @@ struct ep_pqueue {
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};
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/*
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* This semaphore is used to serialize ep_free() and eventpoll_release_file().
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* This mutex is used to serialize ep_free() and eventpoll_release_file().
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*/
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static struct mutex epmutex;
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@ -506,7 +519,7 @@ static void ep_free(struct eventpoll *ep)
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/*
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* We need to lock this because we could be hit by
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* eventpoll_release_file() while we're freeing the "struct eventpoll".
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* We do not need to hold "ep->sem" here because the epoll file
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* We do not need to hold "ep->mtx" here because the epoll file
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* is on the way to be removed and no one has references to it
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* anymore. The only hit might come from eventpoll_release_file() but
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* holding "epmutex" is sufficent here.
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@ -525,7 +538,7 @@ static void ep_free(struct eventpoll *ep)
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/*
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* Walks through the whole tree by freeing each "struct epitem". At this
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* point we are sure no poll callbacks will be lingering around, and also by
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* write-holding "sem" we can be sure that no file cleanup code will hit
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* holding "epmutex" we can be sure that no file cleanup code will hit
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* us during this operation. So we can avoid the lock on "ep->lock".
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*/
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while ((rbp = rb_first(&ep->rbr)) != 0) {
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@ -534,6 +547,8 @@ static void ep_free(struct eventpoll *ep)
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}
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mutex_unlock(&epmutex);
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mutex_destroy(&ep->mtx);
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}
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static int ep_eventpoll_release(struct inode *inode, struct file *file)
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@ -594,9 +609,9 @@ void eventpoll_release_file(struct file *file)
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* We don't want to get "file->f_ep_lock" because it is not
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* necessary. It is not necessary because we're in the "struct file"
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* cleanup path, and this means that noone is using this file anymore.
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* The only hit might come from ep_free() but by holding the semaphore
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* The only hit might come from ep_free() but by holding the mutex
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* will correctly serialize the operation. We do need to acquire
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* "ep->sem" after "epmutex" because ep_remove() requires it when called
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* "ep->mtx" after "epmutex" because ep_remove() requires it when called
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* from anywhere but ep_free().
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*/
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mutex_lock(&epmutex);
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@ -606,9 +621,9 @@ void eventpoll_release_file(struct file *file)
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ep = epi->ep;
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list_del_init(&epi->fllink);
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down_write(&ep->sem);
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mutex_lock(&ep->mtx);
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ep_remove(ep, epi);
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up_write(&ep->sem);
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mutex_unlock(&ep->mtx);
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}
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mutex_unlock(&epmutex);
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@ -622,11 +637,12 @@ static int ep_alloc(struct eventpoll **pep)
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return -ENOMEM;
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rwlock_init(&ep->lock);
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init_rwsem(&ep->sem);
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mutex_init(&ep->mtx);
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init_waitqueue_head(&ep->wq);
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init_waitqueue_head(&ep->poll_wait);
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INIT_LIST_HEAD(&ep->rdllist);
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ep->rbr = RB_ROOT;
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ep->ovflist = EP_UNACTIVE_PTR;
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*pep = ep;
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@ -695,7 +711,21 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
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* until the next EPOLL_CTL_MOD will be issued.
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*/
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if (!(epi->event.events & ~EP_PRIVATE_BITS))
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goto is_disabled;
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goto out_unlock;
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/*
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* If we are trasfering events to userspace, we can hold no locks
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* (because we're accessing user memory, and because of linux f_op->poll()
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* semantics). All the events that happens during that period of time are
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* chained in ep->ovflist and requeued later on.
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*/
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if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
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if (epi->next == EP_UNACTIVE_PTR) {
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epi->next = ep->ovflist;
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ep->ovflist = epi;
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}
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goto out_unlock;
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}
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/* If this file is already in the ready list we exit soon */
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if (ep_is_linked(&epi->rdllink))
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@ -714,7 +744,7 @@ is_linked:
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if (waitqueue_active(&ep->poll_wait))
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pwake++;
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is_disabled:
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out_unlock:
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write_unlock_irqrestore(&ep->lock, flags);
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/* We have to call this outside the lock */
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@ -788,6 +818,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
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epi->event = *event;
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atomic_set(&epi->usecnt, 1);
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epi->nwait = 0;
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epi->next = EP_UNACTIVE_PTR;
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/* Initialize the poll table using the queue callback */
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epq.epi = epi;
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@ -920,36 +951,50 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
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return 0;
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}
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/*
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* This function is called without holding the "ep->lock" since the call to
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* __copy_to_user() might sleep, and also f_op->poll() might reenable the IRQ
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* because of the way poll() is traditionally implemented in Linux.
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*/
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static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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struct epoll_event __user *events, int maxevents)
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static int ep_send_events(struct eventpoll *ep, struct epoll_event __user *events,
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int maxevents)
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{
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int eventcnt, error = -EFAULT, pwake = 0;
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unsigned int revents;
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unsigned long flags;
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struct epitem *epi;
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struct list_head injlist;
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struct epitem *epi, *nepi;
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struct list_head txlist;
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INIT_LIST_HEAD(&injlist);
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INIT_LIST_HEAD(&txlist);
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/*
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* We need to lock this because we could be hit by
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* eventpoll_release_file() and epoll_ctl(EPOLL_CTL_DEL).
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*/
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mutex_lock(&ep->mtx);
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/*
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* Steal the ready list, and re-init the original one to the
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* empty list. Also, set ep->ovflist to NULL so that events
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* happening while looping w/out locks, are not lost. We cannot
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* have the poll callback to queue directly on ep->rdllist,
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* because we are doing it in the loop below, in a lockless way.
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*/
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write_lock_irqsave(&ep->lock, flags);
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list_splice(&ep->rdllist, &txlist);
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INIT_LIST_HEAD(&ep->rdllist);
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ep->ovflist = NULL;
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write_unlock_irqrestore(&ep->lock, flags);
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/*
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* We can loop without lock because this is a task private list.
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* We just splice'd out the ep->rdllist in ep_collect_ready_items().
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* Items cannot vanish during the loop because we are holding "sem" in
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* read.
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* Items cannot vanish during the loop because we are holding "mtx".
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*/
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for (eventcnt = 0; !list_empty(txlist) && eventcnt < maxevents;) {
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epi = list_first_entry(txlist, struct epitem, rdllink);
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prefetch(epi->rdllink.next);
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for (eventcnt = 0; !list_empty(&txlist) && eventcnt < maxevents;) {
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epi = list_first_entry(&txlist, struct epitem, rdllink);
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list_del_init(&epi->rdllink);
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/*
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* Get the ready file event set. We can safely use the file
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* because we are holding the "sem" in read and this will
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* guarantee that both the file and the item will not vanish.
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* because we are holding the "mtx" and this will guarantee
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* that both the file and the item will not vanish.
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*/
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revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
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revents &= epi->event.events;
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@ -957,8 +1002,8 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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/*
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* Is the event mask intersect the caller-requested one,
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* deliver the event to userspace. Again, we are holding
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* "sem" in read, so no operations coming from userspace
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* can change the item.
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* "mtx", so no operations coming from userspace can change
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* the item.
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*/
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if (revents) {
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if (__put_user(revents,
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@ -970,49 +1015,47 @@ static int ep_send_events(struct eventpoll *ep, struct list_head *txlist,
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epi->event.events &= EP_PRIVATE_BITS;
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eventcnt++;
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}
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/*
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* This is tricky. We are holding the "sem" in read, and this
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* means that the operations that can change the "linked" status
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* of the epoll item (epi->rbn and epi->rdllink), cannot touch
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* them. Also, since we are "linked" from a epi->rdllink POV
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* (the item is linked to our transmission list we just
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* spliced), the ep_poll_callback() cannot touch us either,
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* because of the check present in there. Another parallel
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* epoll_wait() will not get the same result set, since we
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* spliced the ready list before. Note that list_del() still
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* shows the item as linked to the test in ep_poll_callback().
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* At this point, noone can insert into ep->rdllist besides
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* us. The epoll_ctl() callers are locked out by us holding
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* "mtx" and the poll callback will queue them in ep->ovflist.
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*/
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list_del(&epi->rdllink);
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if (!(epi->event.events & EPOLLET) &&
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(revents & epi->event.events))
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list_add_tail(&epi->rdllink, &injlist);
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else {
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/*
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* Be sure the item is totally detached before re-init
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* the list_head. After INIT_LIST_HEAD() is committed,
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* the ep_poll_callback() can requeue the item again,
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* but we don't care since we are already past it.
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*/
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smp_mb();
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INIT_LIST_HEAD(&epi->rdllink);
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||||
}
|
||||
(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 @@ 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);
|
||||
|
|
|
|||
Loading…
Reference in a new issue