linux-kernel - [PATCH] compiler.h: Use compound statement for WRITE

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Message-Id: <20220617090423.1743297-1-lucas.demarchi@intel.com>
Date:   Fri, 17 Jun 2022 02:04:24 -0700
From:   Lucas De Marchi <lucas.demarchi@...el.com>
To:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Lucas De Marchi <lucas.demarchi@...el.com>
Subject: [PATCH] compiler.h: Use compound statement for WRITE_ONCE

Instead of a do { } while (0), replace the outer wrapper with compound
statement. It allows to use WRITE_ONCE() inside a _Generic() without
exploding the build with the following error:

	  CC [M]  drivers/gpu/drm/i915/gt/uc/intel_guc_ads.o
	In file included from ./arch/x86/include/generated/asm/rwonce.h:1,
			 from ./include/linux/compiler.h:248,
			 from ./include/linux/build_bug.h:5,
			 from ./include/linux/bitfield.h:10,
			 from ./drivers/gpu/drm/i915/i915_reg_defs.h:9,
			 from ./drivers/gpu/drm/i915/gt/intel_engine_regs.h:9,
			 from drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c:8:
	drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c: In function ‘guc_policies_init’:
	./include/asm-generic/rwonce.h:59:1: error: expected expression before ‘do’
	   59 | do {                                                                    \
	      | ^~
	./include/linux/iosys-map.h:367:13: note: in expansion of macro ‘WRITE_ONCE’
	  367 |         u8: WRITE_ONCE(*((u8 *)vaddr__), val__),                                \
	      |             ^~~~~~~~~~
	./include/linux/iosys-map.h:412:17: note: in expansion of macro ‘__iosys_map_wr_sys’
	  412 |                 __iosys_map_wr_sys(val, (map__)->vaddr + (offset__), type__);   \
	      |                 ^~~~~~~~~~~~~~~~~~
	./include/linux/iosys-map.h:500:9: note: in expansion of macro ‘iosys_map_wr’
	  500 |         iosys_map_wr(map__, struct_offset__ + offsetof(struct_type__, field__),         \
	      |         ^~~~~~~~~~~~
	drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c:68:9: note: in expansion of macro ‘iosys_map_wr_field’
	   68 |         iosys_map_wr_field(&(guc_)->ads_map, 0, struct __guc_ads_blob,  \
	      |         ^~~~~~~~~~~~~~~~~~
	drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c:145:9: note: in expansion of macro ‘ads_blob_write’
	  145 |         ads_blob_write(guc, policies.dpc_promote_time,
	      |         ^~~~~~~~~~~~~~

One alternative would be to wrap the WRITE_ONCE in a compound statement
inside the _Generic(), but there is no downside on simply replacing the
do { } while (0).

Signed-off-by: Lucas De Marchi <lucas.demarchi@...el.com>
---

I hit the above error when adding some additional changes in
include/linux/iosys-map.h. After changing WRITE_ONCE() and checking its
implementation I realized it would be simpler not using a _Generic() for
the WRITE_ONCE, like in
https://lore.kernel.org/dri-devel/20220617085204.1678035-2-lucas.demarchi@intel.com/T/#u

So this patch is not strictly needed, but may be desirable for future
users.

 include/asm-generic/rwonce.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
index 8d0a6280e982..6a94c1f21648 100644
--- a/include/asm-generic/rwonce.h
+++ b/include/asm-generic/rwonce.h
@@ -56,10 +56,10 @@ do {									\
 } while (0)
 
 #define WRITE_ONCE(x, val)						\
-do {									\
+({									\
 	compiletime_assert_rwonce_type(x);				\
 	__WRITE_ONCE(x, val);						\
-} while (0)
+})
 
 static __no_sanitize_or_inline
 unsigned long __read_once_word_nocheck(const void *addr)
-- 
2.36.1