[PATCHv2,1/4] atomic: replace atomic_compareandswap() with atomic_cmpxchg()

Commit Message

David Vrabel Jan. 26, 2016, 4:25 p.m. UTC

atomic_compareandswap() used atomic_t as the new, old and returned
values which is less convinient than using just int.

Signed-off-by: David Vrabel <[e-mail]>
---
v2:
- arm/arm64 already provided atomic_cmpxchg()
---
 xen/common/domain.c          |  5 +----
 xen/include/asm-arm/atomic.h |  8 --------
 xen/include/asm-x86/atomic.h | 24 ++++++++++++++++--------
 xen/include/xen/sched.h      |  9 ++++-----
 4 files changed, 21 insertions(+), 25 deletions(-)

Comments

Ian Campbell Jan. 26, 2016, 5:09 p.m. UTC | #1

On Tue, 2016-01-26 at 16:25 +0000, David Vrabel wrote:
> atomic_compareandswap() used atomic_t as the new, old and returned
> values which is less convinient than using just int.

"convenient"

> diff --git a/xen/include/asm-arm/atomic.h b/xen/include/asm-arm/atomic.h
> index 5a38c67..29ab265 100644
> --- a/xen/include/asm-arm/atomic.h
> +++ b/xen/include/asm-arm/atomic.h
> @@ -138,14 +138,6 @@ static inline void _atomic_set(atomic_t *v, int i)
>  # error "unknown ARM variant"
>  #endif
>  
> -static inline atomic_t atomic_compareandswap(
> -    atomic_t old, atomic_t new, atomic_t *v)
> -{
> -    atomic_t rc;
> -    rc.counter = __cmpxchg(&v->counter, old.counter, new.counter,
> sizeof(int));
> -    return rc;
> -}


Acked-by: Ian Campbell <[e-mail]>

Patch

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 2979c1b..93e77f5 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -857,14 +857,11 @@  static void complete_domain_destroy(struct rcu_head *head)
 void domain_destroy(struct domain *d)
 {
     struct domain **pd;
-    atomic_t old = ATOMIC_INIT(0);
-    atomic_t new = ATOMIC_INIT(DOMAIN_DESTROYED);
 
     BUG_ON(!d->is_dying);
 
     /* May be already destroyed, or get_domain() can race us. */
-    old = atomic_compareandswap(old, new, &d->refcnt);
-    if ( _atomic_read(old) != 0 )
+    if ( atomic_cmpxchg(&d->refcnt, 0, DOMAIN_DESTROYED) != 0 )
         return;
 
     cpupool_rm_domain(d);
diff --git a/xen/include/asm-arm/atomic.h b/xen/include/asm-arm/atomic.h
index 5a38c67..29ab265 100644
--- a/xen/include/asm-arm/atomic.h
+++ b/xen/include/asm-arm/atomic.h
@@ -138,14 +138,6 @@  static inline void _atomic_set(atomic_t *v, int i)
 # error "unknown ARM variant"
 #endif
 
-static inline atomic_t atomic_compareandswap(
-    atomic_t old, atomic_t new, atomic_t *v)
-{
-    atomic_t rc;
-    rc.counter = __cmpxchg(&v->counter, old.counter, new.counter, sizeof(int));
-    return rc;
-}
-
 #endif /* __ARCH_ARM_ATOMIC__ */
 /*
  * Local variables:
diff --git a/xen/include/asm-x86/atomic.h b/xen/include/asm-x86/atomic.h
index 2b8c877..d246b70 100644
--- a/xen/include/asm-x86/atomic.h
+++ b/xen/include/asm-x86/atomic.h
@@ -135,6 +135,10 @@  static inline void _atomic_set(atomic_t *v, int i)
     v->counter = i;
 }
 
+static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
+{
+    return cmpxchg(&v->counter, old, new);
+}
 
 /**
  * atomic_add - add integer to atomic variable
@@ -152,6 +156,18 @@  static inline void atomic_add(int i, atomic_t *v)
 }
 
 /**
+ * atomic_add_return - add integer and return
+ * @i: integer value to add
+ * @v: pointer of type atomic_t
+ *
+ * Atomically adds @i to @v and returns @i + @v
+ */
+static inline int atomic_add_return(int i, atomic_t *v)
+{
+    return i + arch_fetch_and_add(&v->counter, i);
+}
+
+/**
  * atomic_sub - subtract the atomic variable
  * @i: integer value to subtract
  * @v: pointer of type atomic_t
@@ -272,12 +288,4 @@  static inline int atomic_add_negative(int i, atomic_t *v)
     return c;
 }
 
-static inline atomic_t atomic_compareandswap(
-    atomic_t old, atomic_t new, atomic_t *v)
-{
-    atomic_t rc;
-    rc.counter = __cmpxchg(&v->counter, old.counter, new.counter, sizeof(int));
-    return rc;
-}
-
 #endif /* __ARCH_X86_ATOMIC__ */
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 82b6dd1..5870745 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -483,16 +483,15 @@  extern struct vcpu *idle_vcpu[NR_CPUS];
  */
 static always_inline int get_domain(struct domain *d)
 {
-    atomic_t old, new, seen = d->refcnt;
+    int old, seen = atomic_read(&d->refcnt);
     do
     {
         old = seen;
-        if ( unlikely(_atomic_read(old) & DOMAIN_DESTROYED) )
+        if ( unlikely(old & DOMAIN_DESTROYED) )
             return 0;
-        _atomic_set(&new, _atomic_read(old) + 1);
-        seen = atomic_compareandswap(old, new, &d->refcnt);
+        seen = atomic_cmpxchg(&d->refcnt, old, old + 1);
     }
-    while ( unlikely(_atomic_read(seen) != _atomic_read(old)) );
+    while ( unlikely(seen != old) );
     return 1;
 }