[PATCH 1/3] eventfd: Allow waiters to be notified about the eventfdfile* going away

[Gregory Haskins]

From: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>

And give them a change to unregister from the wait queue.

This is turn allows eventfd users to use the eventfd file* w/out holding a
live reference to it.

After the eventfd user callbacks returns, any usage of the eventfd file*
should be dropped.  The eventfd user callback can acquire sleepy locks
since it is invoked lockless.

This is a feature, needed by KVM to avoid an awkward workaround when using
eventfd.

Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
Tested-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
---

 fs/eventfd.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 2a701d5..c71f51d 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -59,7 +59,15 @@ int eventfd_signal(struct file *file, int n)
 
 static int eventfd_release(struct inode *inode, struct file *file)
 {
-	kfree(file->private_data);
+	struct eventfd_ctx *ctx = file->private_data;
+
+	/*
+	 * No need to hold the lock here, since we are on the file cleanup
+	 * path and the ones still attached to the wait queue will be
+	 * serialized by wake_up_locked_poll().
+	 */
+	wake_up_locked_poll(&ctx->wqh, POLLHUP);
+	kfree(ctx);
 	return 0;
 }
 

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