On Wed, Aug 09, 2006 at 05:29:10PM -0700, Andrew Morton wrote:
> On Wed, 9 Aug 2006 17:18:23 -0700
> "Paul E. McKenney" <[email protected]> wrote:
>
> > @@ -129,6 +129,8 @@ unsigned int __kfifo_put(struct kfifo *f
> > /* then put the rest (if any) at the beginning of the buffer */
> > memcpy(fifo->buffer, buffer + l, len - l);
> >
> > + smp_wmb();
> > +
> > fifo->in += len;
> >
> > return len;
> > @@ -161,6 +163,8 @@ unsigned int __kfifo_get(struct kfifo *f
> > /* then get the rest (if any) from the beginning of the buffer */
> > memcpy(buffer + l, fifo->buffer, len - l);
> >
> > + smp_mb();
> > +
> > fifo->out += len;
> >
> > return len;
>
> When adding barriers, please also add a little comment explaining what the
> barrier is protecting us from.
>
> Often it's fairly obvious, but sometimes it is not, and in both cases it is still
> useful to communicate the programmer's intent in this way.
I certainly cannot claim that it was obvious in this case, as the act
of adding the comments caused me to realize that I had added only two
of the four memory barriers that are required. :-/ Updated patch below.
Signed-off-by: Paul E. McKenney <[email protected]>
---
kfifo.c | 28 ++++++++++++++++++++++++++++
1 files changed, 28 insertions(+)
diff -urpNa -X dontdiff linux-2.6.18-rc2/kernel/kfifo.c linux-2.6.18-rc2-kfifo/kernel/kfifo.c
--- linux-2.6.18-rc2/kernel/kfifo.c 2006-07-15 14:53:08.000000000 -0700
+++ linux-2.6.18-rc2-kfifo/kernel/kfifo.c 2006-08-09 17:45:41.000000000 -0700
@@ -122,6 +122,13 @@ unsigned int __kfifo_put(struct kfifo *f
len = min(len, fifo->size - fifo->in + fifo->out);
+ /*
+ * Ensure that we sample the fifo->out index -before- we
+ * start putting bytes into the kfifo.
+ */
+
+ smp_mb();
+
/* first put the data starting from fifo->in to buffer end */
l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));
memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);
@@ -129,6 +136,13 @@ unsigned int __kfifo_put(struct kfifo *f
/* then put the rest (if any) at the beginning of the buffer */
memcpy(fifo->buffer, buffer + l, len - l);
+ /*
+ * Ensure that we add the bytes to the kfifo -before-
+ * we update the fifo->in index.
+ */
+
+ smp_wmb();
+
fifo->in += len;
return len;
@@ -154,6 +168,13 @@ unsigned int __kfifo_get(struct kfifo *f
len = min(len, fifo->in - fifo->out);
+ /*
+ * Ensure that we sample the fifo->in index -before- we
+ * start removing bytes from the kfifo.
+ */
+
+ smp_rmb();
+
/* first get the data from fifo->out until the end of the buffer */
l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));
memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);
@@ -161,6 +182,13 @@ unsigned int __kfifo_get(struct kfifo *f
/* then get the rest (if any) from the beginning of the buffer */
memcpy(buffer + l, fifo->buffer, len - l);
+ /*
+ * Ensure that we remove the bytes from the kfifo -before-
+ * we update the fifo->out index.
+ */
+
+ smp_mb();
+
fifo->out += len;
return len;
-
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