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Subject[RFC][PATCH 5/8] epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll events are ready
Date
From: Arve Hjønnevåg <arve@android.com>

When an epoll_event, that has the EPOLLWAKEUP flag set, is ready, a
wakeup_source will be active to prevent suspend. This can be used to
handle wakeup events from a driver that support poll, e.g. input, if
that driver wakes up the waitqueue passed to epoll before allowing
suspend.

The current implementation uses an extra wakeup_source when
ep_scan_ready_list runs. This can cause problems if a single thread
is polling on wakeup events and frequent non-wakeup events (events
usually arrive during thread freezing) using the same epoll file.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
fs/eventpoll.c | 71 ++++++++++++++++++++++++++++++++++++++++++++--
include/linux/eventpoll.h | 6 +++
2 files changed, 74 insertions(+), 3 deletions(-)
Index: linux/fs/eventpoll.c
===================================================================
--- linux.orig/fs/eventpoll.c
+++ linux/fs/eventpoll.c
@@ -33,6 +33,7 @@
#include <linux/bitops.h>
#include <linux/mutex.h>
#include <linux/anon_inodes.h>
+#include <linux/device.h>
#include <asm/uaccess.h>
#include <asm/io.h>
#include <asm/mman.h>
@@ -87,7 +88,7 @@
*/

/* Epoll private bits inside the event mask */
-#define EP_PRIVATE_BITS (EPOLLONESHOT | EPOLLET)
+#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)

/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4
@@ -154,6 +155,9 @@ struct epitem {
/* List header used to link this item to the "struct file" items list */
struct list_head fllink;

+ /* wakeup_source used when EPOLLWAKEUP is set */
+ struct wakeup_source *ws;
+
/* The structure that describe the interested events and the source fd */
struct epoll_event event;
};
@@ -194,6 +198,9 @@ struct eventpoll {
*/
struct epitem *ovflist;

+ /* wakeup_source used when ep_scan_ready_list is running */
+ struct wakeup_source *ws;
+
/* The user that created the eventpoll descriptor */
struct user_struct *user;

@@ -565,6 +572,7 @@ static int ep_scan_ready_list(struct eve
* in a lockless way.
*/
spin_lock_irqsave(&ep->lock, flags);
+ __pm_stay_awake(ep->ws);
list_splice_init(&ep->rdllist, &txlist);
ep->ovflist = NULL;
spin_unlock_irqrestore(&ep->lock, flags);
@@ -588,8 +596,10 @@ static int ep_scan_ready_list(struct eve
* queued into ->ovflist but the "txlist" might already
* contain them, and the list_splice() below takes care of them.
*/
- if (!ep_is_linked(&epi->rdllink))
+ if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
+ __pm_stay_awake(epi->ws);
+ }
}
/*
* We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
@@ -602,6 +612,7 @@ static int ep_scan_ready_list(struct eve
* Quickly re-inject items left on "txlist".
*/
list_splice(&txlist, &ep->rdllist);
+ __pm_relax(ep->ws);

if (!list_empty(&ep->rdllist)) {
/*
@@ -656,6 +667,9 @@ static int ep_remove(struct eventpoll *e
list_del_init(&epi->rdllink);
spin_unlock_irqrestore(&ep->lock, flags);

+ if (epi->ws)
+ wakeup_source_unregister(epi->ws);
+
/* At this point it is safe to free the eventpoll item */
kmem_cache_free(epi_cache, epi);

@@ -706,6 +720,8 @@ static void ep_free(struct eventpoll *ep
mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
free_uid(ep->user);
+ if (ep->ws)
+ wakeup_source_unregister(ep->ws);
kfree(ep);
}

@@ -737,6 +753,7 @@ static int ep_read_events_proc(struct ev
* callback, but it's not actually ready, as far as
* caller requested events goes. We can remove it here.
*/
+ __pm_relax(epi->ws);
list_del_init(&epi->rdllink);
}
}
@@ -932,8 +949,10 @@ static int ep_poll_callback(wait_queue_t
}

/* If this file is already in the ready list we exit soon */
- if (!ep_is_linked(&epi->rdllink))
+ if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
+ __pm_stay_awake(epi->ws);
+ }

/*
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
@@ -1091,6 +1110,30 @@ static int reverse_path_check(void)
return error;
}

+static int ep_create_wakeup_source(struct epitem *epi)
+{
+ const char *name;
+
+ if (!epi->ep->ws) {
+ epi->ep->ws = wakeup_source_register("eventpoll");
+ if (!epi->ep->ws)
+ return -ENOMEM;
+ }
+
+ name = epi->ffd.file->f_path.dentry->d_name.name;
+ epi->ws = wakeup_source_register(name);
+ if (!epi->ws)
+ return -ENOMEM;
+
+ return 0;
+}
+
+static void ep_destroy_wakeup_source(struct epitem *epi)
+{
+ wakeup_source_unregister(epi->ws);
+ epi->ws = NULL;
+}
+
/*
* Must be called with "mtx" held.
*/
@@ -1118,6 +1161,13 @@ static int ep_insert(struct eventpoll *e
epi->event = *event;
epi->nwait = 0;
epi->next = EP_UNACTIVE_PTR;
+ if (epi->event.events & EPOLLWAKEUP) {
+ error = ep_create_wakeup_source(epi);
+ if (error)
+ goto error_create_wakeup_source;
+ } else {
+ epi->ws = NULL;
+ }

/* Initialize the poll table using the queue callback */
epq.epi = epi;
@@ -1164,6 +1214,7 @@ static int ep_insert(struct eventpoll *e
/* If the file is already "ready" we drop it inside the ready list */
if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
+ __pm_stay_awake(epi->ws);

/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
@@ -1204,6 +1255,10 @@ error_unregister:
list_del_init(&epi->rdllink);
spin_unlock_irqrestore(&ep->lock, flags);

+ if (epi->ws)
+ wakeup_source_unregister(epi->ws);
+
+error_create_wakeup_source:
kmem_cache_free(epi_cache, epi);

return error;
@@ -1229,6 +1284,12 @@ static int ep_modify(struct eventpoll *e
epi->event.events = event->events;
pt._key = event->events;
epi->event.data = event->data; /* protected by mtx */
+ if (epi->event.events & EPOLLWAKEUP) {
+ if (!epi->ws)
+ ep_create_wakeup_source(epi);
+ } else if (epi->ws) {
+ ep_destroy_wakeup_source(epi);
+ }

/*
* Get current event bits. We can safely use the file* here because
@@ -1244,6 +1305,7 @@ static int ep_modify(struct eventpoll *e
spin_lock_irq(&ep->lock);
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
+ __pm_stay_awake(epi->ws);

/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
@@ -1282,6 +1344,7 @@ static int ep_send_events_proc(struct ev
!list_empty(head) && eventcnt < esed->maxevents;) {
epi = list_first_entry(head, struct epitem, rdllink);

+ __pm_relax(epi->ws);
list_del_init(&epi->rdllink);

pt._key = epi->event.events;
@@ -1298,6 +1361,7 @@ static int ep_send_events_proc(struct ev
if (__put_user(revents, &uevent->events) ||
__put_user(epi->event.data, &uevent->data)) {
list_add(&epi->rdllink, head);
+ __pm_stay_awake(epi->ws);
return eventcnt ? eventcnt : -EFAULT;
}
eventcnt++;
@@ -1317,6 +1381,7 @@ static int ep_send_events_proc(struct ev
* poll callback will queue them in ep->ovflist.
*/
list_add_tail(&epi->rdllink, &ep->rdllist);
+ __pm_stay_awake(epi->ws);
}
}
}
Index: linux/include/linux/eventpoll.h
===================================================================
--- linux.orig/include/linux/eventpoll.h
+++ linux/include/linux/eventpoll.h
@@ -26,6 +26,12 @@
#define EPOLL_CTL_DEL 2
#define EPOLL_CTL_MOD 3

+/*
+ * Request the handling of system wakeup events so as to prevent automatic
+ * system suspends from happening while those events are being processed.
+ */
+#define EPOLLWAKEUP (1 << 29)
+
/* Set the One Shot behaviour for the target file descriptor */
#define EPOLLONESHOT (1 << 30)

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