[PATCH] add sched_getcpu, with vDSO support

[Nathan Zadoks <nathan@nathan7.eu>]

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
+  getcpu_f f = (getcpu_f)vdso_func;
+  if (f) {
+    r = f(&cpu, NULL, NULL);
+    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);
+}
-- 
2.7.1