[KVM PATCH v2 1/2] Allow waiters to be notified about the eventfdfile* going away, and give

[Gregory Haskins]

From: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>

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
eventdf.

[gmh: pulled from -mmotm for inclusion in kvm.git]

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>
---

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

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 3f0e197..72f5f8d 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -61,7 +61,15 @@ EXPORT_SYMBOL_GPL(eventfd_signal);
 
 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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