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arm64: barriers: fix smp_load_acquire to work with const arguments

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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>
This commit is contained in:
Will Deacon 2015-11-18 10:13:08 +00:00 committed by Catalin Marinas
parent 0b2aa5b80b
commit c139aa60c1
1 changed files with 10 additions and 6 deletions
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16
arch/arm64/include/asm/barrier.h
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@ -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)