[PATCH v2 1/7] atomics: add acquire/release/relaxed variants of some atomic operations

Will Deacon
2015-07-16T15:32:32Z

Whilst porting the generic qrwlock code over to arm64, it became
apparent that any portable locking code needs finer-grained control of
the memory-ordering guarantees provided by our atomic routines.

In particular: xchg, cmpxchg, {add,sub}_return are often used in
situations where full barrier semantics (currently the only option
available) are not required. For example, when a reader increments a
reader count to obtain a lock, checking the old value to see if a writer
was present, only acquire semantics are strictly needed.

This patch introduces three new ordering semantics for these operations:

- *_relaxed: No ordering guarantees. This is similar to what we have
already for the non-return atomics (e.g. atomic_add).

- *_acquire: ACQUIRE semantics, similar to smp_load_acquire.

- *_release: RELEASE semantics, similar to smp_store_release.

In memory-ordering speak, this means that the acquire/release semantics
are RCpc as opposed to RCsc. Consequently a RELEASE followed by an
ACQUIRE does not imply a full barrier, as already documented in
memory-barriers.txt.

Currently, all the new macros are conditionally mapped to the full-mb
variants, however if the *_relaxed version is provided by the
architecture, then the acquire/release variants are constructed by
supplementing the relaxed routine with an explicit barrier.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
include/linux/atomic.h | 312 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 312 insertions(+)

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 5b08a8540ecf..08c2f6e56f76 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -2,6 +2,318 @@
#ifndef _LINUX_ATOMIC_H
#define _LINUX_ATOMIC_H
#include <asm/atomic.h>
+#include <asm/barrier.h>
+
+/*
+ * Relaxed variants of xchg, cmpxchg and some atomic operations.
+ *
+ * We support four variants:
+ *
+ * - Fully ordered: The default implementation, no suffix required.
+ * - Acquire: Provides ACQUIRE semantics, _acquire suffix.
+ * - Release: Provides RELEASE semantics, _release suffix.
+ * - Relaxed: No ordering guarantees, _relaxed suffix.
+ *
+ * See Documentation/memory-barriers.txt for ACQUIRE/RELEASE definitions.
+ */
+
+#ifndef atomic_read_acquire
+#define atomic_read_acquire(v) smp_load_acquire(&(v)->counter)
+#endif
+
+#ifndef atomic_set_release
+#define atomic_set_release(v, i) smp_store_release(&(v)->counter, (i))
+#endif
+
+/*
+ * The idea here is to build acquire/release variants by adding explicit
+ * barriers on top of the relaxed variant. In the case where the relaxed
+ * variant is already fully ordered, no additional barriers are needed.
+ */
+#define __atomic_op_acquire(ret_t, op, ...) \
+({ \
+ ret_t __ret = op##_relaxed(__VA_ARGS__); \
+ smp_mb__after_atomic(); \
+ __ret; \
+})
+
+#define __atomic_op_release(ret_t, op, ...) \
+({ \
+ ret_t __ret; \
+ smp_mb__before_atomic(); \
+ __ret = op##_relaxed(__VA_ARGS__); \
+ __ret; \
+})
+
+#define __atomic_op_fence(ret_t, op, ...) \
+({ \
+ ret_t __ret; \
+ smp_mb__before_atomic(); \
+ __ret = op##_relaxed(__VA_ARGS__); \
+ smp_mb__after_atomic(); \
+ __ret; \
+})
+
+#ifndef atomic_add_return_relaxed
+#define atomic_add_return_relaxed atomic_add_return
+#define atomic_add_return_acquire atomic_add_return
+#define atomic_add_return_release atomic_add_return
+
+#else /* atomic_add_return_relaxed */
+
+#ifndef atomic_add_return_acquire
+#define atomic_add_return_acquire(...) \
+ __atomic_op_acquire(int, atomic_add_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_add_return_release
+#define atomic_add_return_release(...) \
+ __atomic_op_release(int, atomic_add_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_add_return
+#define atomic_add_return(...) \
+ __atomic_op_fence(int, atomic_add_return, __VA_ARGS__)
+#endif
+#endif /* atomic_add_return_relaxed */
+
+#ifndef atomic_sub_return_relaxed
+#define atomic_sub_return_relaxed atomic_sub_return
+#define atomic_sub_return_acquire atomic_sub_return
+#define atomic_sub_return_release atomic_sub_return
+
+#else /* atomic_sub_return_relaxed */
+
+#ifndef atomic_sub_return_acquire
+#define atomic_sub_return_acquire(...) \
+ __atomic_op_acquire(int, atomic_sub_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_sub_return_release
+#define atomic_sub_return_release(...) \
+ __atomic_op_release(int, atomic_sub_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_sub_return
+#define atomic_sub_return(...) \
+ __atomic_op_fence(int, atomic_sub_return, __VA_ARGS__)
+#endif
+#endif /* atomic_sub_return_relaxed */
+
+#ifndef atomic_xchg_relaxed
+#define atomic_xchg_relaxed atomic_xchg
+#define atomic_xchg_acquire atomic_xchg
+#define atomic_xchg_release atomic_xchg
+
+#else /* atomic_xchg_relaxed */
+
+#ifndef atomic_xchg_acquire
+#define atomic_xchg_acquire(...) \
+ __atomic_op_acquire(int, atomic_xchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic_xchg_release
+#define atomic_xchg_release(...) \
+ __atomic_op_release(int, atomic_xchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic_xchg
+#define atomic_xchg(...) \
+ __atomic_op_fence(int, atomic_xchg, __VA_ARGS__)
+#endif
+#endif /* atomic_xchg_relaxed */
+
+#ifndef atomic_cmpxchg_relaxed
+#define atomic_cmpxchg_relaxed atomic_cmpxchg
+#define atomic_cmpxchg_acquire atomic_cmpxchg
+#define atomic_cmpxchg_release atomic_cmpxchg
+
+#else /* atomic_cmpxchg_relaxed */
+
+#ifndef atomic_cmpxchg_acquire
+#define atomic_cmpxchg_acquire(...) \
+ __atomic_op_acquire(int, atomic_cmpxchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic_cmpxchg_release
+#define atomic_cmpxchg_release(...) \
+ __atomic_op_release(int, atomic_cmpxchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic_cmpxchg
+#define atomic_cmpxchg(...) \
+ __atomic_op_fence(int, atomic_cmpxchg, __VA_ARGS__)
+#endif
+#endif /* atomic_cmpxchg_relaxed */
+
+#ifndef atomic64_read_acquire
+#define atomic64_read_acquire(v) smp_load_acquire(&(v)->counter)
+#endif
+
+#ifndef atomic64_set_release
+#define atomic64_set_release(v, i) smp_store_release(&(v)->counter, (i))
+#endif
+
+#ifndef atomic64_add_return_relaxed
+#define atomic64_add_return_relaxed atomic64_add_return
+#define atomic64_add_return_acquire atomic64_add_return
+#define atomic64_add_return_release atomic64_add_return
+
+#else /* atomic64_add_return_relaxed */
+
+#ifndef atomic64_add_return_acquire
+#define atomic64_add_return_acquire(...) \
+ __atomic_op_acquire(long long, atomic64_add_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_add_return_release
+#define atomic64_add_return_release(...) \
+ __atomic_op_release(long long, atomic64_add_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_add_return
+#define atomic64_add_return(...) \
+ __atomic_op_fence(long long, atomic64_add_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_add_return_relaxed */
+
+#ifndef atomic64_sub_return_relaxed
+#define atomic64_sub_return_relaxed atomic64_sub_return
+#define atomic64_sub_return_acquire atomic64_sub_return
+#define atomic64_sub_return_release atomic64_sub_return
+
+#else /* atomic64_sub_return_relaxed */
+
+#ifndef atomic64_sub_return_acquire
+#define atomic64_sub_return_acquire(...) \
+ __atomic_op_acquire(long long, atomic64_sub_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_sub_return_release
+#define atomic64_sub_return_release(...) \
+ __atomic_op_release(long long, atomic64_sub_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_sub_return
+#define atomic64_sub_return(...) \
+ __atomic_op_fence(long long, atomic64_sub_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_sub_return_relaxed */
+
+#ifndef atomic64_xchg_relaxed
+#define atomic64_xchg_relaxed atomic64_xchg
+#define atomic64_xchg_acquire atomic64_xchg
+#define atomic64_xchg_release atomic64_xchg
+
+#else /* atomic64_xchg_relaxed */
+
+#ifndef atomic64_xchg_acquire
+#define atomic64_xchg_acquire(...) \
+ __atomic_op_acquire(long long, atomic64_xchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_xchg_release
+#define atomic64_xchg_release(...) \
+ __atomic_op_release(long long, atomic64_xchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_xchg
+#define atomic64_xchg(...) \
+ __atomic_op_fence(long long, atomic64_xchg, __VA_ARGS__)
+#endif
+#endif /* atomic64_xchg_relaxed */
+
+#ifndef atomic64_cmpxchg_relaxed
+#define atomic64_cmpxchg_relaxed atomic64_cmpxchg
+#define atomic64_cmpxchg_acquire atomic64_cmpxchg
+#define atomic64_cmpxchg_release atomic64_cmpxchg
+
+#else /* atomic64_cmpxchg_relaxed */
+
+#ifndef atomic64_cmpxchg_acquire
+#define atomic64_cmpxchg_acquire(...) \
+ __atomic_op_acquire(long long, atomic64_cmpxchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_cmpxchg_release
+#define atomic64_cmpxchg_release(...) \
+ __atomic_op_release(long long, atomic64_cmpxchg, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_cmpxchg
+#define atomic64_cmpxchg(...) \
+ __atomic_op_fence(long long, atomic64_cmpxchg, __VA_ARGS__)
+#endif
+#endif /* atomic64_cmpxchg_relaxed */
+
+#ifndef cmpxchg_relaxed
+#define cmpxchg_relaxed cmpxchg
+#define cmpxchg_acquire cmpxchg
+#define cmpxchg_release cmpxchg
+
+#else /* cmpxchg_relaxed */
+
+#ifndef cmpxchg_acquire
+#define cmpxchg_acquire(ptr, ...) \
+ __atomic_op_acquire(__typeof__(*ptr), cmpxchg, ptr, __VA_ARGS__)
+#endif
+
+#ifndef cmpxchg_release
+#define cmpxchg_release(ptr, ...) \
+ __atomic_op_release(__typeof__(*ptr), cmpxchg, ptr, __VA_ARGS__)
+#endif
+
+#ifndef cmpxchg
+#define cmpxchg(ptr, ...) \
+ __atomic_op_fence(__typeof__(*ptr), cmpxchg, ptr, __VA_ARGS__)
+#endif
+#endif /* cmpxchg_relaxed */
+
+#ifndef cmpxchg64_relaxed
+#define cmpxchg64_relaxed cmpxchg64
+#define cmpxchg64_acquire cmpxchg64
+#define cmpxchg64_release cmpxchg64
+
+#else /* cmpxchg64_relaxed */
+
+#ifndef cmpxchg64_acquire
+#define cmpxchg64_acquire(ptr, ...) \
+ __atomic_op_acquire(__typeof__(*ptr), cmpxchg64, ptr, __VA_ARGS__)
+#endif
+
+#ifndef cmpxchg64_release
+#define cmpxchg64_release(ptr, ...) \
+ __atomic_op_release(__typeof__(*ptr), cmpxchg64, ptr, __VA_ARGS__)
+#endif
+
+#ifndef cmpxchg64
+#define cmpxchg64(ptr, ...) \
+ __atomic_op_fence(__typeof__(*ptr), cmpxchg64, ptr, __VA_ARGS__)
+#endif
+#endif /* cmpxchg64_relaxed */
+
+#ifndef xchg_relaxed
+#define xchg_relaxed xchg
+#define xchg_acquire xchg
+#define xchg_release xchg
+
+#else /* xchg_relaxed */
+
+#ifndef xchg_acquire
+#define xchg_acquire(ptr, ...) \
+ __atomic_op_acquire(__typeof__(*ptr), xchg, ptr, __VA_ARGS__)
+#endif
+
+#ifndef xchg_release
+#define xchg_release(ptr, ...) \
+ __atomic_op_release(__typeof__(*ptr), xchg, ptr, __VA_ARGS__)
+#endif
+
+#ifndef xchg
+#define xchg(ptr, ...) \
+ __atomic_op_fence(__typeof__(*ptr), xchg, ptr, __VA_ARGS__)
+#endif
+#endif /* xchg_relaxed */

/**
* atomic_add_unless - add unless the number is already a given value
--
2.1.4