[tip:locking/core] atomic: Implement atomic_read_ctrl()

tip-bot for Peter Zijlstra
2015-09-23T08:49:23Z

Commit-ID: e3e72ab80a3fac0b88e07d358a2c75724ccd66b4
Gitweb: http://git.kernel.org/tip/e3e72ab80a3fac0b88e07d358a2c75724ccd66b4
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 18 Sep 2015 13:22:52 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 23 Sep 2015 09:54:29 +0200

atomic: Implement atomic_read_ctrl()

Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
more conveniently use atomics in control dependencies.

Since we can assume atomic_read() implies a READ_ONCE(), we must only
emit an extra smp_read_barrier_depends() in order to upgrade to
READ_ONCE_CTRL() semantics.

Requested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: oleg@redhat.com
Link: http://lkml.kernel.org/r/20150918115637.GM3604@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
Documentation/memory-barriers.txt | 17 +++++++++--------
include/asm-generic/atomic-long.h | 3 ++-
include/linux/atomic.h | 18 ++++++++++++++++++
3 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 2ba8461..41ffd7e 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -637,7 +637,8 @@ as follows:
b = p; /* BUG: Compiler and CPU can both reorder!!! */

Finally, the READ_ONCE_CTRL() includes an smp_read_barrier_depends()
-that DEC Alpha needs in order to respect control depedencies.
+that DEC Alpha needs in order to respect control depedencies. Alternatively
+use one of atomic{,64}_read_ctrl().

So don't leave out the READ_ONCE_CTRL().

@@ -796,9 +797,9 @@ site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.

In summary:

- (*) Control dependencies must be headed by READ_ONCE_CTRL().
- Or, as a much less preferable alternative, interpose
- smp_read_barrier_depends() between a READ_ONCE() and the
+ (*) Control dependencies must be headed by READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl(). Or, as a much less preferable alternative,
+ interpose smp_read_barrier_depends() between a READ_ONCE() and the
control-dependent write.

(*) Control dependencies can order prior loads against later stores.
@@ -820,10 +821,10 @@ In summary:
and WRITE_ONCE() can help to preserve the needed conditional.

(*) Control dependencies require that the compiler avoid reordering the
- dependency into nonexistence. Careful use of READ_ONCE_CTRL()
- or smp_read_barrier_depends() can help to preserve your control
- dependency. Please see the Compiler Barrier section for more
- information.
+ dependency into nonexistence. Careful use of READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl() or smp_read_barrier_depends() can help to
+ preserve your control dependency. Please see the Compiler Barrier
+ section for more information.

(*) Control dependencies pair normally with other types of barriers.

diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h
index c7a5c1a..8942cdc 100644
--- a/include/asm-generic/atomic-long.h
+++ b/include/asm-generic/atomic-long.h
@@ -35,7 +35,7 @@ typedef atomic_t atomic_long_t;
#endif

#define ATOMIC_LONG_READ_OP(mo) \
-static inline long atomic_long_read##mo(atomic_long_t *l) \
+static inline long atomic_long_read##mo(const atomic_long_t *l) \
{ \
ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l; \
\
@@ -43,6 +43,7 @@ static inline long atomic_long_read##mo(atomic_long_t *l) \
}
ATOMIC_LONG_READ_OP()
ATOMIC_LONG_READ_OP(_acquire)
+ATOMIC_LONG_READ_OP(_ctrl)

#undef ATOMIC_LONG_READ_OP

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 29dafa1..e326469 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -4,6 +4,15 @@
#include <asm/atomic.h>
#include <asm/barrier.h>

+#ifndef atomic_read_ctrl
+static inline int atomic_read_ctrl(const atomic_t *v)
+{
+ int val = atomic_read(v);
+ smp_read_barrier_depends(); /* Enforce control dependency. */
+ return val;
+}
+#endif
+
/*
* Relaxed variants of xchg, cmpxchg and some atomic operations.
*
@@ -455,6 +464,15 @@ static inline int atomic_dec_if_positive(atomic_t *v)
#include <asm-generic/atomic64.h>
#endif

+#ifndef atomic64_read_ctrl
+static inline long long atomic64_read_ctrl(const atomic64_t *v)
+{
+ long long val = atomic64_read(v);
+ smp_read_barrier_depends(); /* Enforce control dependency. */
+ return val;
+}
+#endif
+
#ifndef atomic64_andnot
static inline void atomic64_andnot(long long i, atomic64_t *v)
{