[Xen-devel] [PATCH v3 14/41] asm-generic: add __smp_xxx wrappers

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On !SMP, most architectures define their
barriers as compiler barriers.
On SMP, most need an actual barrier.

Make it possible to remove the code duplication for
!SMP by defining low-level __smp_xxx barriers
which do not depend on the value of SMP, then
use them from asm-generic conditionally.

Besides reducing code duplication, these low level APIs will also be
useful for virtualization, where a barrier is sometimes needed even if
!SMP since we might be talking to another kernel on the same SMP system.

Both virtio and Xen drivers will benefit.

The smp_xxx variants should use __smp_XXX ones or barrier() depending on
SMP, identically for all architectures.

We keep ifndef guards around them for now - once/if all
architectures are converted to use the generic
code, we'll be able to remove these.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 include/asm-generic/barrier.h | 91 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 82 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 987b2e0..8752964 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -54,22 +54,38 @@
 #define read_barrier_depends()         do { } while (0)
 #endif
 
+#ifndef __smp_mb
+#define __smp_mb()     mb()
+#endif
+
+#ifndef __smp_rmb
+#define __smp_rmb()    rmb()
+#endif
+
+#ifndef __smp_wmb
+#define __smp_wmb()    wmb()
+#endif
+
+#ifndef __smp_read_barrier_depends
+#define __smp_read_barrier_depends()   read_barrier_depends()
+#endif
+
 #ifdef CONFIG_SMP
 
 #ifndef smp_mb
-#define smp_mb()       mb()
+#define smp_mb()       __smp_mb()
 #endif
 
 #ifndef smp_rmb
-#define smp_rmb()      rmb()
+#define smp_rmb()      __smp_rmb()
 #endif
 
 #ifndef smp_wmb
-#define smp_wmb()      wmb()
+#define smp_wmb()      __smp_wmb()
 #endif
 
 #ifndef smp_read_barrier_depends
-#define smp_read_barrier_depends()     read_barrier_depends()
+#define smp_read_barrier_depends()     __smp_read_barrier_depends()
 #endif
 
 #else  /* !CONFIG_SMP */
@@ -92,23 +108,78 @@
 
 #endif /* CONFIG_SMP */
 
+#ifndef __smp_store_mb
+#define __smp_store_mb(var, value)  do { WRITE_ONCE(var, value); __smp_mb(); } 
while (0)
+#endif
+
+#ifndef __smp_mb__before_atomic
+#define __smp_mb__before_atomic()      __smp_mb()
+#endif
+
+#ifndef __smp_mb__after_atomic
+#define __smp_mb__after_atomic()       __smp_mb()
+#endif
+
+#ifndef __smp_store_release
+#define __smp_store_release(p, v)                                      \
+do {                                                                   \
+       compiletime_assert_atomic_type(*p);                             \
+       __smp_mb();                                                     \
+       WRITE_ONCE(*p, v);                                              \
+} while (0)
+#endif
+
+#ifndef __smp_load_acquire
+#define __smp_load_acquire(p)                                          \
+({                                                                     \
+       typeof(*p) ___p1 = READ_ONCE(*p);                               \
+       compiletime_assert_atomic_type(*p);                             \
+       __smp_mb();                                                     \
+       ___p1;                                                          \
+})
+#endif
+
+#ifdef CONFIG_SMP
+
+#ifndef smp_store_mb
+#define smp_store_mb(var, value)  __smp_store_mb(var, value)
+#endif
+
+#ifndef smp_mb__before_atomic
+#define smp_mb__before_atomic()        __smp_mb__before_atomic()
+#endif
+
+#ifndef smp_mb__after_atomic
+#define smp_mb__after_atomic() __smp_mb__after_atomic()
+#endif
+
+#ifndef smp_store_release
+#define smp_store_release(p, v) __smp_store_release(p, v)
+#endif
+
+#ifndef smp_load_acquire
+#define smp_load_acquire(p) __smp_load_acquire(p)
+#endif
+
+#else  /* !CONFIG_SMP */
+
 #ifndef smp_store_mb
-#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); smp_mb(); } 
while (0)
+#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); barrier(); } 
while (0)
 #endif
 
 #ifndef smp_mb__before_atomic
-#define smp_mb__before_atomic()        smp_mb()
+#define smp_mb__before_atomic()        barrier()
 #endif
 
 #ifndef smp_mb__after_atomic
-#define smp_mb__after_atomic() smp_mb()
+#define smp_mb__after_atomic() barrier()
 #endif
 
 #ifndef smp_store_release
 #define smp_store_release(p, v)                                                
\
 do {                                                                   \
        compiletime_assert_atomic_type(*p);                             \
-       smp_mb();                                                       \
+       barrier();                                                      \
        WRITE_ONCE(*p, v);                                              \
 } while (0)
 #endif
@@ -118,10 +189,12 @@ do {                                                      
                \
 ({                                                                     \
        typeof(*p) ___p1 = READ_ONCE(*p);                               \
        compiletime_assert_atomic_type(*p);                             \
-       smp_mb();                                                       \
+       barrier();                                                      \
        ___p1;                                                          \
 })
 #endif
 
+#endif
+
 #endif /* !__ASSEMBLY__ */
 #endif /* __ASM_GENERIC_BARRIER_H */
-- 
MST


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