[PATCH] cachefiles: pass a poll key when waking the daemon

[Max Kellermann]

Keyless wakeups bypass epoll's event-mask filtering.  With only
`EPOLLOUT` registered, any cachefiles_state_changed() call will wake
up the epoll_wait() system call, but will not return any events,
causing a busy loop.

Pass the relevant poll events with each wakeup so `epoll` can filter
out unwanted notifications.

Fixes: a5b3a80b899b ("CacheFiles: Provide read-and-reset release counters for cachefilesd")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
---
Note: backports to older branches need to adjust ondemand.c as well
like this:
-       wake_up_all(&cache->daemon_pollwq);
+       cachefiles_wake_poll(cache, EPOLLIN);
---
 fs/cachefiles/cache.c    |  4 ++--
 fs/cachefiles/internal.h | 15 +++++++++++++--
 fs/cachefiles/namei.c    |  2 +-
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/cachefiles/cache.c b/fs/cachefiles/cache.c
index 9fb06dc16520..08ca598618ac 100644
--- a/fs/cachefiles/cache.c
+++ b/fs/cachefiles/cache.c
@@ -255,7 +255,7 @@ int cachefiles_has_space(struct cachefiles_cache *cache,
 	    test_and_clear_bit(CACHEFILES_CULLING, &cache->flags)
 	    ) {
 		_debug("cease culling");
-		cachefiles_state_changed(cache);
+		cachefiles_state_changed(cache, EPOLLIN);
 	}
 
 	//_leave(" = 0");
@@ -275,7 +275,7 @@ int cachefiles_has_space(struct cachefiles_cache *cache,
 begin_cull:
 	if (!test_and_set_bit(CACHEFILES_CULLING, &cache->flags)) {
 		_debug("### CULL CACHE ###");
-		cachefiles_state_changed(cache);
+		cachefiles_state_changed(cache, EPOLLIN | EPOLLOUT);
 	}
 
 	_leave(" = %d", ret);
diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index c93324e0f98c..b8a5ea4a5ced 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -15,6 +15,7 @@
 #include <linux/fscache-cache.h>
 #include <linux/cred.h>
 #include <linux/security.h>
+#include <linux/poll.h>
 
 #define CACHEFILES_DIO_BLOCK_SIZE 4096
 
@@ -117,13 +118,23 @@ struct cachefiles_object *cachefiles_cres_object(struct netfs_cache_resources *c
 	return fscache_cres_cookie(cres)->cache_priv;
 }
 
+/*
+ * Wake up the daemon polling on /dev/cachefiles.
+ */
+static inline void cachefiles_wake_poll(struct cachefiles_cache *cache,
+					__poll_t mask)
+{
+	__wake_up(&cache->daemon_pollwq, TASK_NORMAL, 0, poll_to_key(mask));
+}
+
 /*
  * note change of state for daemon
  */
-static inline void cachefiles_state_changed(struct cachefiles_cache *cache)
+static inline void cachefiles_state_changed(struct cachefiles_cache *cache,
+					    __poll_t mask)
 {
 	set_bit(CACHEFILES_STATE_CHANGED, &cache->flags);
-	wake_up_all(&cache->daemon_pollwq);
+	cachefiles_wake_poll(cache, mask);
 }
 
 /*
diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 88955249a1a6..52eb227f07e8 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -74,7 +74,7 @@ void cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
 	if (!test_bit(CACHEFILES_OBJECT_USING_TMPFILE, &object->flags)) {
 		atomic_long_add(inode->i_blocks, &cache->b_released);
 		if (atomic_inc_return(&cache->f_released))
-			cachefiles_state_changed(cache);
+			cachefiles_state_changed(cache, EPOLLIN);
 	}
 }
 
-- 
2.47.3