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

From: Davide Libenzi <davidel@xmailserver.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org
Subject: Re: [PATCH 0/2] eventfd: new EFD_STATE flag
Date: Sun, 10 Jan 2010 11:04:23 -0800 (PST)	[thread overview]
Message-ID: <alpine.DEB.2.00.1001101102250.20563@makko.or.mcafeemobile.com> (raw)
In-Reply-To: <20100110173557.GA29350@redhat.com>

On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:

> On Sun, Jan 10, 2010 at 09:27:34AM -0800, Davide Libenzi wrote:
> > On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> > 
> > > Not sure what you mean by irqfd locking.  IMO we must take waitqueue
> > > lock, otherwise we can not prevent wait queue callback from being
> > > invoked, right? Note that if we take our own lock under wait queue
> > > callback, we won't be able to use this lock around remove_wait_queue.
> > > Also note how fs/evetfd.c uses __remove_wait_queue after taking wait
> > > queue lock - we'd like to do the same with our wait queue.
> > > 
> > > Could you clarify pls?
> > 
> > Wait a second. Looking at the current code in Linus tree, when you 
> > deassign an irqfd, you are actually destroying it, so the idea of saving 
> > the counter on deassign is not going to work.
> > 
> > 
> > 
> > - Davide
> 
> Yes, the only context passed between deassign and assign is the eventfd
> itself. So I think the following code for deassign would work provided
> eventfd_ctx_read_locked works with wait queue lock taken:
> 
>         spin_lock_irqsave(&irqfd->wqh.lock, flags);
>         eventfd_ctx_read_locked(ctx->eventfd, 1, &ucnt);
>         __remove_wait_queue(irqfd->wqh, &irqfd->wait);
>         spin_lock_irqrestore(&irqfd->wqh.lock, flags);

As I said, you cannot do that from KVM, since write-witers will nedd to be 
woke up.
Use the eventfd_ctx_remove_wait_queue() below (NOTE: compiled, not tested).



- Davide


---
 fs/eventfd.c            |   88 +++++++++++++++++++++++++++++++++++++++---------
 include/linux/eventfd.h |   15 ++++++++
 2 files changed, 88 insertions(+), 15 deletions(-)

Index: linux-2.6.mod/fs/eventfd.c
===================================================================

--- linux-2.6.mod.orig/fs/eventfd.c	2010-01-06 18:33:27.000000000 -0800
+++ linux-2.6.mod/fs/eventfd.c	2010-01-10 11:02:00.000000000 -0800
@@ -135,26 +135,71 @@ static unsigned int eventfd_poll(struct 
 	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 
 		__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)
Index: linux-2.6.mod/include/linux/eventfd.h
===================================================================
--- linux-2.6.mod.orig/include/linux/eventfd.h	2010-01-06 18:33:27.000000000 -0800
+++ linux-2.6.mod/include/linux/eventfd.h	2010-01-10 10:57:06.000000000 -0800
@@ -34,6 +34,9 @@ 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 +64,18 @@ static inline void eventfd_ctx_put(struc
 
 }
 
+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 */
