linux-kernel - [PATCH] [2/6] kfifo: Make kfifo

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Date:	Sun, 27 Dec 2009 22:03:12 +0100 (CET)
From:	Andi Kleen <andi@...stfloor.org>
To:	stefani@...bold.net, linux-kernel@...r.kernel.org, akpm@...l.org
Subject: [PATCH] [2/6] kfifo: Make kfifo_in atomic


Right now kfifo_in allows copying in less than the input
amount. This is unfortunately not a good idea on any
record oriented users: if the size of the kfifo is not
a multiple of the record (and that can easily happen due
to the power-of-two requirement) then when the FIFO fills
up partial records could be put in. Such a condition would
be fatal for any record consumer who would get permanently
desynchronized. In fact I doubt unless the input 
is a totally boundary less data stream I doubt anything
could handle this.

Change kfifo_in() to always put in everything or nothing.

The return value is now always 0 or the full length.

Signed-off-by: Andi Kleen <ak@...ux.intel.com>

---
 kernel/kfifo.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

Index: linux/kernel/kfifo.c
===================================================================
--- linux.orig/kernel/kfifo.c
+++ linux/kernel/kfifo.c
@@ -228,9 +228,8 @@ EXPORT_SYMBOL(__kfifo_in_n);
  * @from: the data to be added.
  * @len: the length of the data to be added.
  *
- * This function copies at most @len bytes from the @from buffer into
- * the FIFO depending on the free space, and returns the number of
- * bytes copied.
+ * This function copies @len bytes from the @from buffer into
+ * the FIFO and returns 0 if there is not enough space.
  *
  * Note that with only one concurrent reader and one concurrent
  * writer, you don't need extra locking to use these functions.
@@ -238,8 +237,8 @@ EXPORT_SYMBOL(__kfifo_in_n);
 unsigned int kfifo_in(struct kfifo *fifo, const void *from,
 				unsigned int len)
 {
-	len = min(kfifo_avail(fifo), len);
-
+	if (kfifo_avail(fifo) < len)
+		return 0;
 	__kfifo_in_data(fifo, from, len, 0);
 	__kfifo_add_in(fifo, len);
 	return len;
--
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