[PATCH -v2 3/6] locking: Introduce smp_acquire__after_ctrl_dep

Peter Zijlstra
2016-05-26T14:26:57Z

[-- Attachment #1: peterz-locking-smp_acquire__after_ctrl_dep.patch --]
[-- Type: text/plain, Size: 2747 bytes --]

Introduce smp_acquire__after_ctrl_dep(), this construct is not
uncommen, but the lack of this barrier is.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/compiler.h | 15 ++++++++++-----
ipc/sem.c | 14 ++------------
2 files changed, 12 insertions(+), 17 deletions(-)

--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -305,6 +305,15 @@ static __always_inline void __write_once
})

/**
+ * smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency
+ *
+ * A control dependency provides a LOAD->STORE order, the additional RMB
+ * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
+ * aka. (load)-ACQUIRE.
+ */
+#define smp_acquire__after_ctrl_dep() smp_rmb()
+
+/**
* cmpwait - compare and wait for a variable to change
* @ptr: pointer to the variable to wait on
* @val: the value it should change from
@@ -331,10 +340,6 @@ static __always_inline void __write_once
*
* Due to C lacking lambda expressions we load the value of *ptr into a
* pre-named variable @VAL to be used in @cond.
- *
- * The control dependency provides a LOAD->STORE order, the additional RMB
- * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
- * aka. ACQUIRE.
*/
#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, cond_expr) ({ \
@@ -346,7 +351,7 @@ static __always_inline void __write_once
break; \
cmpwait(__PTR, VAL); \
} \
- smp_rmb(); /* ctrl + rmb := acquire */ \
+ smp_acquire__after_ctrl_dep(); \
VAL; \
})
#endif
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head
}

/*
- * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
- * are only control barriers.
- * The code must pair with spin_unlock(&sem->lock) or
- * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient.
- *
- * smp_rmb() is sufficient, as writes cannot pass the control barrier.
- */
-#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb()
-
-/*
* Wait until all currently ongoing simple ops have completed.
* Caller must own sem_perm.lock.
* New simple ops cannot start, because simple ops first check
@@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
}
- ipc_smp_acquire__after_spin_is_unlocked();
+ smp_acquire__after_ctrl_dep();
}

/*
@@ -350,7 +340,7 @@ static inline int sem_lock(struct sem_ar
* complex_count++;
* spin_unlock(sem_perm.lock);
*/
- ipc_smp_acquire__after_spin_is_unlocked();
+ smp_acquire__after_ctrl_dep();

/*
* Now repeat the test of complex_count: