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SubjectRe: arm64 tools build failure wrt smp_load_{acquire,release} expansion on gcc version 5.4.0 20160609 (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.9)
Hi Arnaldo,

On Wed, Oct 31, 2018 at 12:45:50PM -0300, Arnaldo Carvalho de Melo wrote:
> So I noticed the following build failure thare point to:
>
> commit 09d62154f61316f7e97eae3f31ef8770c7e4b386
> Author: Daniel Borkmann <daniel@iogearbox.net>
> Date: Fri Oct 19 15:51:02 2018 +0200
>
> tools, perf: add and use optimized ring_buffer_{read_head, write_tail} helpers
>
> -------------------------
>
> 50 ubuntu:16.04-x-arm64 : FAIL aarch64-linux-gnu-gcc (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
>
> Works well with:
>
> 59 ubuntu:18.04-x-arm64 : Ok aarch64-linux-gnu-gcc (Ubuntu/Linaro 7.3.0-27ubuntu1~18.04) 7.3.0
>
> And all the other environments I test build :-)

Cheers for reporting this. I managed to reproduce the build failure with
gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1).

The code in question is the arm64 versions of smp_load_acquire() and
smp_store_release(). Unlike other architectures, these are not built
around READ_ONCE() and WRITE_ONCE() since we have instructions we can
use instead of fences. Bringing our macros up-to-date with those (i.e.
tweaking the union initialisation and using the special "uXX_alias_t"
types) appears to fix the issue for me.

Diff below...

Will

--->8

diff --git a/tools/arch/arm64/include/asm/barrier.h b/tools/arch/arm64/include/asm/barrier.h
index 12835ea0e417..378c051fa177 100644
--- a/tools/arch/arm64/include/asm/barrier.h
+++ b/tools/arch/arm64/include/asm/barrier.h
@@ -14,74 +14,75 @@
#define wmb() asm volatile("dmb ishst" ::: "memory")
#define rmb() asm volatile("dmb ishld" ::: "memory")

-#define smp_store_release(p, v) \
-do { \
- union { typeof(*p) __val; char __c[1]; } __u = \
- { .__val = (__force typeof(*p)) (v) }; \
- \
- switch (sizeof(*p)) { \
- case 1: \
- asm volatile ("stlrb %w1, %0" \
- : "=Q" (*p) \
- : "r" (*(__u8 *)__u.__c) \
- : "memory"); \
- break; \
- case 2: \
- asm volatile ("stlrh %w1, %0" \
- : "=Q" (*p) \
- : "r" (*(__u16 *)__u.__c) \
- : "memory"); \
- break; \
- case 4: \
- asm volatile ("stlr %w1, %0" \
- : "=Q" (*p) \
- : "r" (*(__u32 *)__u.__c) \
- : "memory"); \
- break; \
- case 8: \
- asm volatile ("stlr %1, %0" \
- : "=Q" (*p) \
- : "r" (*(__u64 *)__u.__c) \
- : "memory"); \
- break; \
- default: \
- /* Only to shut up gcc ... */ \
- mb(); \
- break; \
- } \
+#define smp_store_release(p, v) \
+do { \
+ union { typeof(*p) __val; char __c[1]; } __u = \
+ { .__val = (v) }; \
+ \
+ switch (sizeof(*p)) { \
+ case 1: \
+ asm volatile ("stlrb %w1, %0" \
+ : "=Q" (*p) \
+ : "r" (*(__u8_alias_t *)__u.__c) \
+ : "memory"); \
+ break; \
+ case 2: \
+ asm volatile ("stlrh %w1, %0" \
+ : "=Q" (*p) \
+ : "r" (*(__u16_alias_t *)__u.__c) \
+ : "memory"); \
+ break; \
+ case 4: \
+ asm volatile ("stlr %w1, %0" \
+ : "=Q" (*p) \
+ : "r" (*(__u32_alias_t *)__u.__c) \
+ : "memory"); \
+ break; \
+ case 8: \
+ asm volatile ("stlr %1, %0" \
+ : "=Q" (*p) \
+ : "r" (*(__u64_alias_t *)__u.__c) \
+ : "memory"); \
+ break; \
+ default: \
+ /* Only to shut up gcc ... */ \
+ mb(); \
+ break; \
+ } \
} while (0)

-#define smp_load_acquire(p) \
-({ \
- union { typeof(*p) __val; char __c[1]; } __u; \
- \
- switch (sizeof(*p)) { \
- case 1: \
- asm volatile ("ldarb %w0, %1" \
- : "=r" (*(__u8 *)__u.__c) \
- : "Q" (*p) : "memory"); \
- break; \
- case 2: \
- asm volatile ("ldarh %w0, %1" \
- : "=r" (*(__u16 *)__u.__c) \
- : "Q" (*p) : "memory"); \
- break; \
- case 4: \
- asm volatile ("ldar %w0, %1" \
- : "=r" (*(__u32 *)__u.__c) \
- : "Q" (*p) : "memory"); \
- break; \
- case 8: \
- asm volatile ("ldar %0, %1" \
- : "=r" (*(__u64 *)__u.__c) \
- : "Q" (*p) : "memory"); \
- break; \
- default: \
- /* Only to shut up gcc ... */ \
- mb(); \
- break; \
- } \
- __u.__val; \
+#define smp_load_acquire(p) \
+({ \
+ union { typeof(*p) __val; char __c[1]; } __u = \
+ { .__c = { 0 } }; \
+ \
+ switch (sizeof(*p)) { \
+ case 1: \
+ asm volatile ("ldarb %w0, %1" \
+ : "=r" (*(__u8_alias_t *)__u.__c) \
+ : "Q" (*p) : "memory"); \
+ break; \
+ case 2: \
+ asm volatile ("ldarh %w0, %1" \
+ : "=r" (*(__u16_alias_t *)__u.__c) \
+ : "Q" (*p) : "memory"); \
+ break; \
+ case 4: \
+ asm volatile ("ldar %w0, %1" \
+ : "=r" (*(__u32_alias_t *)__u.__c) \
+ : "Q" (*p) : "memory"); \
+ break; \
+ case 8: \
+ asm volatile ("ldar %0, %1" \
+ : "=r" (*(__u64_alias_t *)__u.__c) \
+ : "Q" (*p) : "memory"); \
+ break; \
+ default: \
+ /* Only to shut up gcc ... */ \
+ mb(); \
+ break; \
+ } \
+ __u.__val; \
})

#endif /* _TOOLS_LINUX_ASM_AARCH64_BARRIER_H */
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