[tip:locking/core] locking/atomic: Add atomic_cond_read_acquire()

[tip-bot for Will Deacon]

Commit-ID:  4df714be4dcf40bfb0d4af0f851a6e1977afa02e
Gitweb:     https://git.kernel.org/tip/4df714be4dcf40bfb0d4af0f851a6e1977afa02e
Author:     Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Thu, 12 Oct 2017 13:20:48 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 25 Oct 2017 10:57:24 +0200

locking/atomic: Add atomic_cond_read_acquire()

smp_cond_load_acquire() provides a way to spin on a variable with acquire
semantics until some conditional expression involving the variable is
satisfied. Architectures such as arm64 can potentially enter a low-power
state, waking up only when the value of the variable changes, which
reduces the system impact of tight polling loops.

This patch makes the same interface available to users of atomic_t,
atomic64_t and atomic_long_t, rather than require messy accesses to the
structure internals.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Jeremy.Linton@xxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1507810851-306-3-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/asm-generic/atomic-long.h | 3 +++
 include/linux/atomic.h            | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h
index 288cc9e..f2d97b7 100644
--- a/include/asm-generic/atomic-long.h
+++ b/include/asm-generic/atomic-long.h
@@ -243,4 +243,7 @@ static inline long atomic_long_add_unless(atomic_long_t *l, long a, long u)
 #define atomic_long_inc_not_zero(l) \
 	ATOMIC_LONG_PFX(_inc_not_zero)((ATOMIC_LONG_PFX(_t) *)(l))
 
+#define atomic_long_cond_read_acquire(v, c) \
+	ATOMIC_LONG_PFX(_cond_read_acquire)((ATOMIC_LONG_PFX(_t) *)(v), (c))
+
 #endif  /*  _ASM_GENERIC_ATOMIC_LONG_H  */
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 40d6bfe..0aeb2b3 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -653,6 +653,8 @@ static inline int atomic_dec_if_positive(atomic_t *v)
 }
 #endif
 
+#define atomic_cond_read_acquire(v, c)	smp_cond_load_acquire(&(v)->counter, (c))
+
 #ifdef CONFIG_GENERIC_ATOMIC64
 #include <asm-generic/atomic64.h>
 #endif
@@ -1072,6 +1074,8 @@ static inline long long atomic64_fetch_andnot_release(long long i, atomic64_t *v
 }
 #endif
 
+#define atomic64_cond_read_acquire(v, c)	smp_cond_load_acquire(&(v)->counter, (c))
+
 #include <asm-generic/atomic-long.h>
 
 #endif /* _LINUX_ATOMIC_H */