Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

[Davide Libenzi <davidel@xmailserver.org>]

On Sat, 20 Jun 2009, Davide Libenzi wrote:

> On Sat, 20 Jun 2009, Davide Libenzi wrote:
> 
> > How about the one below?
> 
> Maybe with an interface that can be undone w/out a file* :)

This is another alternative, based on a low-carb diet of your notifier 
patch.
Same concept of de-coupling VFS refcount from eventfd memory context, and 
allowing a poll callback register/unregister.
AFAICS, based on my limited knowledge of the IRQfd policies, your 
->release() path needs to eventfd_pollcb_unregister() and wait for all 
pending works to be done.




- Davide



---
 fs/eventfd.c            |   69 +++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/eventfd.h |   23 ++++++++++++++++
 2 files changed, 91 insertions(+), 1 deletion(-)

Index: linux-2.6.mod/fs/eventfd.c
===================================================================
--- linux-2.6.mod.orig/fs/eventfd.c	2009-06-20 16:25:45.000000000 -0700
+++ linux-2.6.mod/fs/eventfd.c	2009-06-20 16:35:22.000000000 -0700
@@ -17,8 +17,10 @@
 #include <linux/eventfd.h>
 #include <linux/syscalls.h>
 #include <linux/module.h>
+#include <linux/kref.h>
 
 struct eventfd_ctx {
+	struct kref kref;
 	wait_queue_head_t wqh;
 	/*
 	 * Every time that a write(2) is performed on an eventfd, the
@@ -59,9 +61,29 @@ int eventfd_signal(struct file *file, in
 }
 EXPORT_SYMBOL_GPL(eventfd_signal);
 
+static void eventfd_free(struct kref *kref)
+{
+	struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
+
+	kfree(ctx);
+}
+
+static void eventfd_get(struct eventfd_ctx *ctx)
+{
+	kref_get(&ctx->kref);
+}
+
+static void eventfd_put(struct eventfd_ctx *ctx)
+{
+	kref_put(&ctx->kref, eventfd_free);
+}
+
 static int eventfd_release(struct inode *inode, struct file *file)
 {
-	kfree(file->private_data);
+	struct eventfd_ctx *ctx = file->private_data;
+
+	wake_up_poll(&ctx->wqh, POLLHUP);
+	eventfd_put(ctx);
 	return 0;
 }
 
@@ -217,6 +239,7 @@ SYSCALL_DEFINE2(eventfd2, unsigned int, 
 	if (!ctx)
 		return -ENOMEM;
 
+	kref_init(&ctx->kref);
 	init_waitqueue_head(&ctx->wqh);
 	ctx->count = count;
 	ctx->flags = flags;
@@ -237,3 +260,47 @@ SYSCALL_DEFINE1(eventfd, unsigned int, c
 	return sys_eventfd2(count, 0);
 }
 
+static void eventfd_pollcb_ptqueue(struct file *file, wait_queue_head_t *wqh,
+				   poll_table *pt)
+{
+	struct eventfd_pollcb *ecb;
+
+	ecb = container_of(pt, struct eventfd_pollcb, pt);
+
+	add_wait_queue(wqh, &ecb->wait);
+}
+
+int eventfd_pollcb_register(struct file *file, struct eventfd_pollcb *ecb,
+			    wait_queue_func_t cbf)
+{
+	struct eventfd_ctx *ctx;
+	unsigned int events;
+
+	if (file->f_op != &eventfd_fops)
+		return -EINVAL;
+
+	ctx = file->private_data;
+
+	/*
+	 * Install our own custom wake-up handling so we are notified via
+	 * a callback whenever someone signals the underlying eventfd.
+	 */
+	init_waitqueue_func_entry(&ecb->wait, cbf);
+	init_poll_funcptr(&ecb->pt, eventfd_pollcb_ptqueue);
+
+	events = file->f_op->poll(file, &ecb->pt);
+
+	eventfd_get(ctx);
+	ecb->ctx = ctx;
+
+	return (events & POLLIN) ? 1 : 0;
+}
+EXPORT_SYMBOL_GPL(eventfd_pollcb_register);
+
+void eventfd_pollcb_unregister(struct eventfd_pollcb *ecb)
+{
+	remove_wait_queue(&ecb->ctx->wqh, &ecb->wait);
+	eventfd_put(ecb->ctx);
+}
+EXPORT_SYMBOL_GPL(eventfd_pollcb_unregister);
+
Index: linux-2.6.mod/include/linux/eventfd.h
===================================================================
--- linux-2.6.mod.orig/include/linux/eventfd.h	2009-06-20 16:25:45.000000000 -0700
+++ linux-2.6.mod/include/linux/eventfd.h	2009-06-20 16:38:20.000000000 -0700
@@ -8,6 +8,20 @@
 #ifndef _LINUX_EVENTFD_H
 #define _LINUX_EVENTFD_H
 
+#include <linux/wait.h>
+#include <linux/poll.h>
+#include <linux/file.h>
+#include <linux/list.h>
+#include <linux/kref.h>
+
+struct eventfd_ctx;
+
+struct eventfd_pollcb {
+	poll_table pt;
+	struct eventfd_ctx *ctx;
+	wait_queue_t wait;
+};
+
 #ifdef CONFIG_EVENTFD
 
 /* For O_CLOEXEC and O_NONBLOCK */
@@ -29,12 +43,21 @@
 
 struct file *eventfd_fget(int fd);
 int eventfd_signal(struct file *file, int n);
+int eventfd_pollcb_register(struct file *file, struct eventfd_pollcb *ecb,
+			    wait_queue_func_t cbf);
+void eventfd_pollcb_unregister(struct eventfd_pollcb *ecb);
 
 #else /* CONFIG_EVENTFD */
 
 #define eventfd_fget(fd) ERR_PTR(-ENOSYS)
 static inline int eventfd_signal(struct file *file, int n)
 { return 0; }
+static inline int eventfd_pollcb_register(struct file *file,
+					  struct eventfd_pollcb *ecb,
+					  wait_queue_func_t cbf)
+{ return -ENOSYS; }
+static inline void eventfd_pollcb_unregister(struct eventfd_pollcb *ecb)
+{ }
 
 #endif /* CONFIG_EVENTFD */