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Subject[PATCH 05/13] alpha: Override READ_ONCE() with barriered implementation
Date
Rather then relying on the core code to use 'smp_read_barrier_depends()'
as part of the 'READ_ONCE()' definition, instead override 'READ_ONCE()'
in the Alpha code so that it is treated the same way as
'smp_load_acquire()'.

Signed-off-by: Will Deacon <will@kernel.org>
---
arch/alpha/include/asm/barrier.h | 61 ++++----------------------------
arch/alpha/include/asm/rwonce.h | 22 ++++++++++++
2 files changed, 29 insertions(+), 54 deletions(-)
create mode 100644 arch/alpha/include/asm/rwonce.h

diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
index 92ec486a4f9e..1f6abe2d1392 100644
--- a/arch/alpha/include/asm/barrier.h
+++ b/arch/alpha/include/asm/barrier.h
@@ -2,64 +2,17 @@
#ifndef __BARRIER_H
#define __BARRIER_H

-#include <asm/compiler.h>
-
#define mb() __asm__ __volatile__("mb": : :"memory")
#define rmb() __asm__ __volatile__("mb": : :"memory")
#define wmb() __asm__ __volatile__("wmb": : :"memory")

-/**
- * read_barrier_depends - Flush all pending reads that subsequents reads
- * depend on.
- *
- * No data-dependent reads from memory-like regions are ever reordered
- * over this barrier. All reads preceding this primitive are guaranteed
- * to access memory (but not necessarily other CPUs' caches) before any
- * reads following this primitive that depend on the data return by
- * any of the preceding reads. This primitive is much lighter weight than
- * rmb() on most CPUs, and is never heavier weight than is
- * rmb().
- *
- * These ordering constraints are respected by both the local CPU
- * and the compiler.
- *
- * Ordering is not guaranteed by anything other than these primitives,
- * not even by data dependencies. See the documentation for
- * memory_barrier() for examples and URLs to more information.
- *
- * For example, the following code would force ordering (the initial
- * value of "a" is zero, "b" is one, and "p" is "&a"):
- *
- * <programlisting>
- * CPU 0 CPU 1
- *
- * b = 2;
- * memory_barrier();
- * p = &b; q = p;
- * read_barrier_depends();
- * d = *q;
- * </programlisting>
- *
- * because the read of "*q" depends on the read of "p" and these
- * two reads are separated by a read_barrier_depends(). However,
- * the following code, with the same initial values for "a" and "b":
- *
- * <programlisting>
- * CPU 0 CPU 1
- *
- * a = 2;
- * memory_barrier();
- * b = 3; y = b;
- * read_barrier_depends();
- * x = a;
- * </programlisting>
- *
- * does not enforce ordering, since there is no data dependency between
- * the read of "a" and the read of "b". Therefore, on some CPUs, such
- * 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 read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
+#define __smp_load_acquire(p) \
+({ \
+ typeof(*p) ___p1 = (*(volatile typeof(*p) *)(p)); \
+ compiletime_assert_atomic_type(*p); \
+ mb(); \
+ ___p1; \
+})

#ifdef CONFIG_SMP
#define __ASM_SMP_MB "\tmb\n"
diff --git a/arch/alpha/include/asm/rwonce.h b/arch/alpha/include/asm/rwonce.h
new file mode 100644
index 000000000000..ef5601352b55
--- /dev/null
+++ b/arch/alpha/include/asm/rwonce.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2019 Google LLC.
+ */
+#ifndef __ASM_RWONCE_H
+#define __ASM_RWONCE_H
+
+#include <asm/barrier.h>
+
+/*
+ * Alpha is apparently daft enough to reorder address-dependent loads
+ * on some CPU implementations. Knock some common sense into it with
+ * a memory barrier in READ_ONCE().
+ */
+#define __read_once_size_1(p) __smp_load_acquire((u8 *)(p))
+#define __read_once_size_2(p) __smp_load_acquire((u16 *)(p))
+#define __read_once_size_4(p) __smp_load_acquire((u32 *)(p))
+#define __read_once_size_8(p) __smp_load_acquire((u64 *)(p))
+
+#include <asm-generic/rwonce.h>
+
+#endif /* __ASM_RWONCE_H */
--
2.24.0.rc1.363.gb1bccd3e3d-goog
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