arm64: barriers: fix smp_load_acquire to work with const arguments

A newly introduced function in include/net/sock.h passes a const
argument to smp_load_acquire:

  static inline int sk_state_load(const struct sock *sk)
  {
	return smp_load_acquire(&sk->sk_state);
  }

This cause an allmodconfig build failure, since our underlying
load-acquire implementation does not handle const types correctly:

  include/net/sock.h: In function 'sk_state_load':
  ./arch/arm64/include/asm/barrier.h:71:3: error: read-only variable '___p1' used as 'asm' output
     asm volatile ("ldarb %w0, %1"    \

This patch fixes the problem by reusing the trick in READ_ONCE that
loads via a non-const member of an anonymous union. This has the
advantage of allowing us to use smp_load_acquire on packed structures
(e.g. arch_spinlock_t) as well as primitive types.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Daney <david.daney@cavium.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: David Daney <david.daney@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 624f967..9622eb4 100644 (file)
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -64,27 +64,31 @@ do {                                                                        \
 
 #define smp_load_acquire(p)                                            \
 ({                                                                     \
-       typeof(*p) ___p1;                                               \
+       union { typeof(*p) __val; char __c[1]; } __u;                   \
        compiletime_assert_atomic_type(*p);                             \
        switch (sizeof(*p)) {                                           \
        case 1:                                                         \
                asm volatile ("ldarb %w0, %1"                           \
-                       : "=r" (___p1) : "Q" (*p) : "memory");          \
+                       : "=r" (*(__u8 *)__u.__c)                       \
+                       : "Q" (*p) : "memory");                         \
                break;                                                  \
        case 2:                                                         \
                asm volatile ("ldarh %w0, %1"                           \
-                       : "=r" (___p1) : "Q" (*p) : "memory");          \
+                       : "=r" (*(__u16 *)__u.__c)                      \
+                       : "Q" (*p) : "memory");                         \
                break;                                                  \
        case 4:                                                         \
                asm volatile ("ldar %w0, %1"                            \
-                       : "=r" (___p1) : "Q" (*p) : "memory");          \
+                       : "=r" (*(__u32 *)__u.__c)                      \
+                       : "Q" (*p) : "memory");                         \
                break;                                                  \
        case 8:                                                         \
                asm volatile ("ldar %0, %1"                             \
-                       : "=r" (___p1) : "Q" (*p) : "memory");          \
+                       : "=r" (*(__u64 *)__u.__c)                      \
+                       : "Q" (*p) : "memory");                         \
                break;                                                  \
        }                                                               \
-       ___p1;                                                          \
+       __u.__val;                                                      \
 })
 
 #define read_barrier_depends()         do { } while(0)