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From:   Dennis Zhou <dennis@kernel.org>
To:     Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dennis Zhou <dennis@kernel.org>,
        kernel test robot <lkp@intel.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: [PATCH] percpu: fix clang modpost section mismatch
Date:   Sun, 14 Feb 2021 18:18:02 +0000
Message-Id: <20210214181802.896248-1-dennis@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

pcpu_build_alloc_info() is an __init function that makes a call to
cpumask_clear_cpu(). With CONFIG_GCOV_PROFILE_ALL enabled, the inline
heuristics are modified and such cpumask_clear_cpu() which is marked
inline doesn't get inlined. Because it works on mask in __initdata,
modpost throws a section mismatch error.

Arnd sent a patch with the flatten attribute as an alternative [2]. I've
added it to compiler_attributes.h.

modpost complaint:
  WARNING: modpost: vmlinux.o(.text+0x735425): Section mismatch in reference from the function cpumask_clear_cpu() to the variable .init.data:pcpu_build_alloc_info.mask
  The function cpumask_clear_cpu() references
  the variable __initdata pcpu_build_alloc_info.mask.
  This is often because cpumask_clear_cpu lacks a __initdata
  annotation or the annotation of pcpu_build_alloc_info.mask is wrong.

clang output:
  mm/percpu.c:2724:5: remark: cpumask_clear_cpu not inlined into pcpu_build_alloc_info because too costly to inline (cost=725, threshold=325) [-Rpass-missed=inline]

[1] https://lore.kernel.org/linux-mm/202012220454.9F6Bkz9q-lkp@intel.com/
[2] https://lore.kernel.org/lkml/CAK8P3a2ZWfNeXKSm8K_SUhhwkor17jFo3xApLXjzfPqX0eUDUA@mail.gmail.com/

Reported-by: kernel test robot <lkp@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
 include/linux/compiler_attributes.h | 6 ++++++
 mm/percpu.c                         | 2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index ea5e04e75845..c043b8d2b17b 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -210,6 +210,12 @@
 # define fallthrough                    do {} while (0)  /* fallthrough */
 #endif
 
+/*
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#flatten
+ */
+# define __flatten			__attribute__((flatten))
+
 /*
  * Note the missing underscores.
  *
diff --git a/mm/percpu.c b/mm/percpu.c
index 80f8f885a990..6596a0a4286e 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -2663,7 +2663,7 @@ early_param("percpu_alloc", percpu_alloc_setup);
  * On success, pointer to the new allocation_info is returned.  On
  * failure, ERR_PTR value is returned.
  */
-static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
+static struct pcpu_alloc_info * __init __flatten pcpu_build_alloc_info(
 				size_t reserved_size, size_t dyn_size,
 				size_t atom_size,
 				pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
-- 
2.30.0.478.g8a0d178c01-goog