Re: [rcu:rcu/next 30/45] include/linux/compiler.h:343:2: error: implicit declaration of function 'smp_read_barrier_depends'

[Will Deacon]

Hi Paul,

It looks like the breakage below is from a version of the patches that
existed before I split compiler.h in half. What's the plan with these
patches? I'd be happier for you to take them, but you'll want to take the
most recent version in that case.

Cheers,

Will

On Sun, Oct 15, 2017 at 07:29:36AM +0800, kbuild test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/next
> head:   ea788e7da4886dff9782ad61d4c5e6ebadfa8260
> commit: c3a030152f67ef977129c11e5b37a8e6071d4b6f [30/45] locking/barriers: Kill lockless_dereference
> config: sparc-alldefconfig (attached as .config)
> compiler: sparc-linux-gcc (GCC) 6.2.0
> reproduce:
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         git checkout c3a030152f67ef977129c11e5b37a8e6071d4b6f
>         # save the attached .config to linux build tree
>         make.cross ARCH=sparc 
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from include/uapi/linux/stddef.h:1:0,
>                     from include/linux/stddef.h:4,
>                     from include/uapi/linux/posix_types.h:4,
>                     from include/uapi/linux/types.h:13,
>                     from include/linux/types.h:5,
>                     from include/linux/thread_info.h:10,
>                     from arch/sparc/include/asm/current.h:14,
>                     from include/linux/sched.h:11,
>                     from arch/sparc/kernel/asm-offsets.c:13:
>    include/linux/list.h: In function 'list_empty':
> >> include/linux/compiler.h:343:2: error: implicit declaration of function 'smp_read_barrier_depends' [-Werror=implicit-function-declaration]
>      smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
>      ^
>    include/linux/compiler.h:346:22: note: in expansion of macro '__READ_ONCE'
>     #define READ_ONCE(x) __READ_ONCE(x, 1)
>                          ^~~~~~~~~~~
>    include/linux/list.h:202:9: note: in expansion of macro 'READ_ONCE'
>      return READ_ONCE(head->next) == head;
>             ^~~~~~~~~
>    cc1: some warnings being treated as errors
>    make[2]: *** [arch/sparc/kernel/asm-offsets.s] Error 1
>    make[2]: Target '__build' not remade because of errors.
>    make[1]: *** [prepare0] Error 2
>    make[1]: Target 'prepare' not remade because of errors.
>    make: *** [sub-make] Error 2
> 
> vim +/smp_read_barrier_depends +343 include/linux/compiler.h
> 
>    312	
>    313	/*
>    314	 * Prevent the compiler from merging or refetching reads or writes. The
>    315	 * compiler is also forbidden from reordering successive instances of
>    316	 * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
>    317	 * compiler is aware of some particular ordering.  One way to make the
>    318	 * compiler aware of ordering is to put the two invocations of READ_ONCE,
>    319	 * WRITE_ONCE or ACCESS_ONCE() in different C statements.
>    320	 *
>    321	 * In contrast to ACCESS_ONCE these two macros will also work on aggregate
>    322	 * data types like structs or unions. If the size of the accessed data
>    323	 * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
>    324	 * READ_ONCE() and WRITE_ONCE() will fall back to memcpy(). There's at
>    325	 * least two memcpy()s: one for the __builtin_memcpy() and then one for
>    326	 * the macro doing the copy of variable - '__u' allocated on the stack.
>    327	 *
>    328	 * Their two major use cases are: (1) Mediating communication between
>    329	 * process-level code and irq/NMI handlers, all running on the same CPU,
>    330	 * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
>    331	 * mutilate accesses that either do not require ordering or that interact
>    332	 * with an explicit memory barrier or atomic instruction that provides the
>    333	 * required ordering.
>    334	 */
>    335	
>    336	#define __READ_ONCE(x, check)						\
>    337	({									\
>    338		union { typeof(x) __val; char __c[1]; } __u;			\
>    339		if (check)							\
>    340			__read_once_size(&(x), __u.__c, sizeof(x));		\
>    341		else								\
>    342			__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
>  > 343		smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
>    344		__u.__val;							\
>    345	})
>    346	#define READ_ONCE(x) __READ_ONCE(x, 1)
>    347	
> 
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