On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote:

On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:

quoted

+/**
+ * volatile_if() - Provide a control-dependency
+ *
+ * volatile_if(READ_ONCE(A))
+ *	WRITE_ONCE(B, 1);
+ *
+ * will ensure that the STORE to B happens after the LOAD of A. Normally a
+ * control dependency relies on a conditional branch having a data dependency
+ * on the LOAD and an architecture's inability to speculate STOREs. IOW, this
+ * provides a LOAD->STORE order.
+ *
+ * Due to optimizing compilers extra care is needed; as per the example above
+ * the LOAD must be 'volatile' qualified in order to ensure the compiler
+ * actually emits the load, such that the data-dependency to the conditional
+ * branch can be formed.
+ *
+ * Secondly, the compiler must be prohibited from lifting anything out of the
+ * selection statement, as this would obviously also break the ordering.
+ *
+ * Thirdly, and this is the tricky bit, architectures that allow the
+ * LOAD->STORE reorder must ensure the compiler actually emits the conditional
+ * branch instruction, this isn't possible in generic.
+ *
+ * See the volatile_cond() wrapper.
+ */
+#define volatile_if(cond) if (volatile_cond(cond))

On naming (sorry Paul for forgetting that in the initial mail); while I
think using the volatile qualifier for the language feature (can we haz
plz, kthxbai) makes perfect sense, Paul felt that we might use a
'better' name for the kernel use, ctrl_dep_if() was proposed.

Let us pain bike sheds :-)

I have felt that pain many times...  ;-)

Here is what I see thus far from these two threads:

1.	volatile_if() as above.  Nice ease of use, but might be suboptimal
	on architectures where a branch is slower than an acquire load.

2.	#1, but with my preferred name of ctrl_dep_if() instead of
	volatile_if().

3.	READ_ONCE_CTRL() like back in the old days.  This has the
	advantage of giving the compiler more information, but has
	problems with relaxed atomic RMW operations.

4.	A full (fool?) solution based on #3 would also include _ctrl
	suffixed atomic RMW operations.

5.	Your bikeshed color here!

							Thanx, Paul