[glibc/google/grte/v5-2.27/master] Add workaround for infinite looping in ppc vsyscall for sched_getcpu.

[Stan Shebs <shebs@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=973a510a9408ec58d5c3216ce411757ba6c6e489

commit 973a510a9408ec58d5c3216ce411757ba6c6e489
Author: Stan Shebs <stanshebs@google.com>
Date:   Fri Jan 24 08:17:38 2020 -0800

    Add workaround for infinite looping in ppc vsyscall for sched_getcpu.

Diff:
---
 sysdeps/unix/sysv/linux/sched_getcpu.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index b69eeda..2279026 100644
--- a/sysdeps/unix/sysv/linux/sched_getcpu.c
+++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
@@ -24,11 +24,28 @@
 #endif
 #include <sysdep-vdso.h>
 
+#if defined __clang__ && defined __powerpc64__
+/* On ppc, sched_getcpu's body eventually expands into asm code
+   that does a bctrl, but clang does not recognize the need to save
+   the link register, so calls loop infinitely instead of returning.
+   As workaround, make a dummy function call that forces a link
+   register save.  */
+volatile int sched_getcpu_dummy_glob;
+
+void __attribute__((noinline)) sched_getcpu_dummy ()
+{
+  sched_getcpu_dummy_glob = 45;
+}
+#endif
+
 int
 sched_getcpu (void)
 {
 #ifdef __NR_getcpu
   unsigned int cpu;
+#if defined __clang__ && defined __powerpc64__
+  sched_getcpu_dummy ();
+#endif
   int r = INLINE_VSYSCALL (getcpu, 3, &cpu, NULL, NULL);
 
   return r == -1 ? r : cpu;