Re: [PATCH 0/2] eventfd: new EFD_STATE flag

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 11, 2010 at 02:53:53PM -0800, Davide Libenzi wrote:
> On Mon, 11 Jan 2010, Michael S. Tsirkin wrote:
> 
> > On Mon, Jan 11, 2010 at 11:14:14AM -0800, Davide Libenzi wrote:
> > > On Mon, 11 Jan 2010, Michael S. Tsirkin wrote:
> > > 
> > > > Yes, I think this will work. Will test and report. Thanks!
> > > 
> > > If you ever happen to find a solution in KVM that does not require eventfd 
> > > changes, that'd be even better :)
> > > Otherwise ping me when you tested, that I'll puch this to Andrew.
> > > 
> > > 
> > > - Davide
> > 
> > Hmm, the fix also needs changes in kvm driver to call the new API. To
> > simplify dependencies, can we merge this patch through Avi's kvm tree as
> > well?
> 
> It is fine for me. I can officially post it to Avi, CCing kvm and lkml.
> 
> 
> - Davide

Since it now uses wait_queue_t, I had to add include linux/wait.h to
make it self-contained.  The following works fine for me. Thanks!

--->

From: Davide Libenzi <davidel@xmailserver.org>
Subject: eventfd: support eventfd_ctx_remove_wait_queue

Add eventfd_ctx_remove_wait_queue API so that kernel users can clear
counter while getting out of poll queue, atomically.

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Tested-by: Michael S. Tsirkin <mst@redhat.com>

--

diff --git a/fs/eventfd.c b/fs/eventfd.c
index d26402f..4cda278 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -135,26 +135,71 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait)
 	return events;
 }
 
-static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
-			    loff_t *ppos)
+static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
+{
+	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1: ctx->count;
+	ctx->count -= *cnt;
+}
+
+/**
+ * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
+ * @ctx: [in] Pointer to eventfd context.
+ * @wait: [in] Wait queue to be removed.
+ * @cnt: [out] Pointer to the 64bit conter value.
+ *
+ * Returns zero if successful, or the following error codes:
+ *
+ * -EAGAIN      : The operation would have blocked.
+ *
+ * This is used to atomically remove a wait queue entry from the eventfd wait
+ * queue head, and read/reset the counter value.
+ */
+int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
+				  __u64 *cnt)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&ctx->wqh.lock, flags);
+	eventfd_ctx_do_read(ctx, cnt);
+	__remove_wait_queue(&ctx->wqh, wait);
+	if (*cnt != 0 && waitqueue_active(&ctx->wqh))
+		wake_up_locked_poll(&ctx->wqh, POLLOUT);
+	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+
+	return *cnt != 0 ? 0: -EAGAIN;
+}
+EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
+
+/**
+ * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
+ * @ctx: [in] Pointer to eventfd context.
+ * @no_wait: [in] Different from zero if the operation should not block.
+ * @cnt: [out] Pointer to the 64bit conter value.
+ *
+ * Returns zero if successful, or the following error codes:
+ *
+ * -EAGAIN      : The operation would have blocked but @no_wait was nonzero.
+ * -ERESTARTSYS : A signal interrupted the wait operation.
+ *
+ * If @no_wait is zero, the function might sleep until the eventfd internal
+ * counter becomes greater than zero.
+ */
+ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
 {
-	struct eventfd_ctx *ctx = file->private_data;
 	ssize_t res;
-	__u64 ucnt = 0;
 	DECLARE_WAITQUEUE(wait, current);
 
-	if (count < sizeof(ucnt))
-		return -EINVAL;
 	spin_lock_irq(&ctx->wqh.lock);
+	*cnt = 0;
 	res = -EAGAIN;
 	if (ctx->count > 0)
-		res = sizeof(ucnt);
-	else if (!(file->f_flags & O_NONBLOCK)) {
+		res = 0;
+	else if (!no_wait) {
 		__add_wait_queue(&ctx->wqh, &wait);
-		for (res = 0;;) {
+		for (;;) {
 			set_current_state(TASK_INTERRUPTIBLE);
 			if (ctx->count > 0) {
-				res = sizeof(ucnt);
+				res = 0;
 				break;
 			}
 			if (signal_pending(current)) {
@@ -168,18 +213,31 @@ static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
 		__remove_wait_queue(&ctx->wqh, &wait);
 		__set_current_state(TASK_RUNNING);
 	}
-	if (likely(res > 0)) {
-		ucnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
-		ctx->count -= ucnt;
+	if (likely(res == 0)) {
+		eventfd_ctx_do_read(ctx, cnt);
 		if (waitqueue_active(&ctx->wqh))
 			wake_up_locked_poll(&ctx->wqh, POLLOUT);
 	}
 	spin_unlock_irq(&ctx->wqh.lock);
-	if (res > 0 && put_user(ucnt, (__u64 __user *) buf))
-		return -EFAULT;
 
 	return res;
 }
+EXPORT_SYMBOL_GPL(eventfd_ctx_read);
+
+static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
+			    loff_t *ppos)
+{
+	struct eventfd_ctx *ctx = file->private_data;
+	ssize_t res;
+	__u64 cnt;
+
+	if (count < sizeof(cnt))
+		return -EINVAL;
+	if ((res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt)) < 0)
+		return res;
+
+	return put_user(cnt, (__u64 __user *) buf) ? -EFAULT: sizeof(cnt);
+}
 
 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
 			     loff_t *ppos)
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index 94dd103..91bb4f2 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -10,6 +10,7 @@
 
 #include <linux/fcntl.h>
 #include <linux/file.h>
+#include <linux/wait.h>
 
 /*
  * CAREFUL: Check include/asm-generic/fcntl.h when defining
@@ -34,6 +35,9 @@ struct file *eventfd_fget(int fd);
 struct eventfd_ctx *eventfd_ctx_fdget(int fd);
 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file);
 int eventfd_signal(struct eventfd_ctx *ctx, int n);
+ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt);
+int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
+				  __u64 *cnt);
 
 #else /* CONFIG_EVENTFD */
 
@@ -61,6 +65,18 @@ static inline void eventfd_ctx_put(struct eventfd_ctx *ctx)
 
 }
 
+static inline ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait,
+				       __u64 *cnt)
+{
+	return -ENOSYS;
+}
+
+static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
+						wait_queue_t *wait, __u64 *cnt)
+{
+	return -ENOSYS;
+}
+
 #endif
 
 #endif /* _LINUX_EVENTFD_H */