Re: [PATCH] add sched_getcpu, with vDSO support

[Rich Felker <dalias@libc.org>]

On Tue, Mar 01, 2016 at 04:56:06PM +0100, Nathan Zadoks wrote:
> This is a GNU extension, but a fairly minor one, for a system call that
> otherwise has no libc wrapper.
> 
> Adding it was discussed previously, without any objections:
> http://www.openwall.com/lists/musl/2015/05/08/24
> ---
> 
> Whoops, forgot to make getcpu_init a static function in the previous version.
> Fixed!
> 
> ---
> 
>  arch/x86_64/syscall_arch.h |  2 ++
>  include/sched.h            |  1 +
>  src/sched/sched_getcpu.c   | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 48 insertions(+)
>  create mode 100644 src/sched/sched_getcpu.c
> 
> diff --git a/arch/x86_64/syscall_arch.h b/arch/x86_64/syscall_arch.h
> index a7a7b5a..54e05ff 100644
> --- a/arch/x86_64/syscall_arch.h
> +++ b/arch/x86_64/syscall_arch.h
> @@ -64,3 +64,5 @@ static __inline long __syscall6(long n, long a1, long a2, long a3, long a4, long
>  #define VDSO_USEFUL
>  #define VDSO_CGT_SYM "__vdso_clock_gettime"
>  #define VDSO_CGT_VER "LINUX_2.6"
> +#define VDSO_GETCPU_SYM "__vdso_getcpu"
> +#define VDSO_GETCPU_VER "LINUX_2.6"
> diff --git a/include/sched.h b/include/sched.h
> index 3e34a72..7e88f09 100644
> --- a/include/sched.h
> +++ b/include/sched.h
> @@ -76,6 +76,7 @@ void free(void *);
>  
>  typedef struct cpu_set_t { unsigned long __bits[128/sizeof(long)]; } cpu_set_t;
>  int __sched_cpucount(size_t, const cpu_set_t *);
> +int sched_getcpu(void);
>  int sched_getaffinity(pid_t, size_t, cpu_set_t *);
>  int sched_setaffinity(pid_t, size_t, const cpu_set_t *);
>  
> diff --git a/src/sched/sched_getcpu.c b/src/sched/sched_getcpu.c
> new file mode 100644
> index 0000000..096f06f
> --- /dev/null
> +++ b/src/sched/sched_getcpu.c
> @@ -0,0 +1,45 @@
> +#define _GNU_SOURCE
> +#include <stdlib.h>
> +#include <errno.h>
> +#include <sched.h>
> +#include "syscall.h"
> +#include "atomic.h"
> +
> +#ifdef VDSO_GETCPU_SYM
> +
> +void *__vdsosym(const char *, const char *);
> +
> +static void *volatile vdso_func;
> +
> +typedef long (*getcpu_f)(unsigned *, unsigned *, void *);
> +
> +static long getcpu_init(unsigned *cpu, unsigned *node, void *unused)
> +{
> +  void *p = __vdsosym(VDSO_GETCPU_VER, VDSO_GETCPU_SYM);
> +  getcpu_f f = (getcpu_f)p;
> +  a_cas_p(&vdso_func, (void *)getcpu_init, p);
> +  return f ? f(cpu, node, unused) : -ENOSYS;
> +}
> +
> +static void *volatile vdso_func = (void *)getcpu_init;
> +
> +#endif
> +
> +int sched_getcpu(void)
> +{
> +  int r;
> +  unsigned cpu;
> +
> +#ifdef VDSO_CGT_SYM

Wrong macro in the #ifdef.

> +  getcpu_f f = (getcpu_f)vdso_func;
> +  if (f) {
> +    r = f(&cpu, NULL, NULL);

Not a big deal, but usually in musl we use 0 rather than the NULL
macro.

Actually I wondered if the function actually needs to take the useless
extra 2 arguments, but I think for the sake of correctness it's best
to do it this way -- the callee in the vdso has 3 args, so it should
be called with the correct type.

> +    if (!r) return cpu;
> +    if (r != -ENOSYS) return __syscall_ret(r);
> +  }
> +#endif
> +
> +  r = __syscall(SYS_getcpu, &cpu, NULL, NULL);
> +  if (!r) return cpu;
> +  return __syscall_ret(r);
> +}

Also tabs should be used for indention, not spaces.

One other thing I thought might be nice is initially committing the
trivial syscall-only version that adds the public prototype, then
doing vdso support as a separate commit, but if that's a pain to do
don't worry about it.

Rich