On Wed, Oct 07, 2015 at 01:10:24PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 06, 2015 at 06:04:50PM +0200, Peter Zijlstra wrote:
> > That said, it would be good if Paul (or anyone really) can explain to me
> > the reason for: 5af4692a75da ("smp: Make control dependencies work on
> > Alpha, improve documentation"). The Changelog simply states that Alpha
> > needs the mb, but not how/why etc.
"Alpha AXP Architecture Reference Manual, Second Edition", Sites & Witek,
page 5-20, Section 5.6.3, "Implied Barriers":
In Alpha AXP, there are no implied barriers. If an implied
barrier is needed for functionally correct access to shared
data, it must be written as an explicit instruction. (Software
must explicitly include any needed MB, WMB, or CALL_PAL IMB
instructions.)
On exactly how the hardware might choose not to respect control
dependencies, I must defer to someone who knows about the Alpha hardware.
On my last opportunity to discuss this with Alpha architects, I was
concerned only about data dependencies, and didn't think to ask about
control dependencies.
> Also, I suppose Documentation/circular-buffer.txt needs help.
Who wrote that??? ;-)
How about the following?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/Documentation/circular-buffers.txt
b/Documentation/circular-buffers.txt
index 88951b179262..c71c0cab7bbb 100644
--- a/Documentation/circular-buffers.txt
+++ b/Documentation/circular-buffers.txt
@@ -161,7 +161,7 @@ The producer will look something like this:
unsigned long head = buffer->head;
/* The spin_unlock() and next spin_lock() provide needed ordering. */
- unsigned long tail = ACCESS_ONCE(buffer->tail);
+ unsigned long tail = READ_ONCE_CTRL(buffer->tail);
if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
/* insert one item into the buffer */
@@ -222,7 +222,7 @@ This will instruct the CPU to make sure the index is up to
date before reading
the new item, and then it shall make sure the CPU has finished reading the item
before it writes the new tail pointer, which will erase the item.
-Note the use of ACCESS_ONCE() and smp_load_acquire() to read the
+Note the use of READ_ONCE_CTRL() and smp_load_acquire() to read the
opposition index. This prevents the compiler from discarding and
reloading its cached value - which some compilers will do across
smp_read_barrier_depends(). This isn't strictly needed if you can
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