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Subject[PATCH RFC 3/4] barriers: convert a control to a data dependency
It's not uncommon to have two access two unrelated memory locations in a
specific order. At the moment one has to use a memory barrier for this.

However, if the first access was a read and the second used an address
depending on the first one we would have a data dependency and no
barrier would be necessary.

This adds a new interface: dependent_ptr_mb which does exactly this: it
returns a pointer with a data dependency on the supplied value.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
Documentation/memory-barriers.txt | 20 ++++++++++++++++++++
arch/alpha/include/asm/barrier.h | 1 +
include/asm-generic/barrier.h | 18 ++++++++++++++++++
include/linux/compiler.h | 4 ++++
4 files changed, 43 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index c1d913944ad8..9dbaa2e1dbf6 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -691,6 +691,18 @@ case what's actually required is:
p = READ_ONCE(b);
}

+Alternatively, a control dependency can be converted to a data dependency,
+e.g.:
+
+ q = READ_ONCE(a);
+ if (q) {
+ b = dependent_ptr_mb(b, q);
+ p = READ_ONCE(b);
+ }
+
+Note how the result of dependent_ptr_mb must be used with the following
+accesses in order to have an effect.
+
However, stores are not speculated. This means that ordering -is- provided
for load-store control dependencies, as in the following example:

@@ -836,6 +848,12 @@ out-guess your code. More generally, although READ_ONCE() does force
the compiler to actually emit code for a given load, it does not force
the compiler to use the results.

+Converting to a data dependency helps with this too:
+
+ q = READ_ONCE(a);
+ b = dependent_ptr_mb(b, q);
+ WRITE_ONCE(b, 1);
+
In addition, control dependencies apply only to the then-clause and
else-clause of the if-statement in question. In particular, it does
not necessarily apply to code following the if-statement:
@@ -875,6 +893,8 @@ to the CPU containing it. See the section on "Multicopy atomicity"
for more information.


+
+
In summary:

(*) Control dependencies can order prior loads against later stores.
diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
index 92ec486a4f9e..b4934e8c551b 100644
--- a/arch/alpha/include/asm/barrier.h
+++ b/arch/alpha/include/asm/barrier.h
@@ -59,6 +59,7 @@
* as Alpha, "y" could be set to 3 and "x" to 0. Use rmb()
* in cases like this where there are no data dependencies.
*/
+#define ARCH_NEEDS_READ_BARRIER_DEPENDS 1
#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")

#ifdef CONFIG_SMP
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 2cafdbb9ae4c..fa2e2ef72b68 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -70,6 +70,24 @@
#define __smp_read_barrier_depends() read_barrier_depends()
#endif

+#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \
+ !defined(ARCH_NEEDS_READ_BARRIER_DEPENDS)
+
+#define dependent_ptr_mb(ptr, val) ({ \
+ long dependent_ptr_mb_val = (long)(val); \
+ long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val; \
+ \
+ BUILD_BUG_ON(sizeof(val) > sizeof(long)); \
+ OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val); \
+ (typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val); \
+})
+
+#else
+
+#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); })
+
+#endif
+
#ifdef CONFIG_SMP

#ifndef smp_mb
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 6601d39e8c48..f599c30f1b28 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -152,9 +152,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
#endif

#ifndef OPTIMIZER_HIDE_VAR
+
/* Make the optimizer believe the variable can be manipulated arbitrarily. */
#define OPTIMIZER_HIDE_VAR(var) \
__asm__ ("" : "=rm" (var) : "0" (var))
+
+#define COMPILER_HAS_OPTIMIZER_HIDE_VAR 1
+
#endif

/* Not-quite-unique ID. */
--
MST
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