[PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

[Florian Weimer <fweimer@redhat.com>]

Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
Unconditionally use a 32-byte rseq area").  After that, it was
not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
compilation that provided a definition.  This commit always checks
the rseq area for CPU number information before using the other
approaches.

This adds an unnecessary (but well-predictable) branch on
architectures which do not define RSEQ_SIG, but its cost is small
compared to the system call.  Most architectures that have vDSO
acceleration for getcpu also have rseq support.

Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
Fixes: 1d350aa06091211863e41169729cee1bca39f72f

---
v3: Remove #ifdef RSEQ_SIG instead of making it defined.
 sysdeps/unix/sysv/linux/sched_getcpu.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index dfb884568d..72a3360550 100644
--- a/sysdeps/unix/sysv/linux/sched_getcpu.c
+++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
@@ -33,17 +33,9 @@ vsyscall_sched_getcpu (void)
   return r == -1 ? r : cpu;
 }
 
-#ifdef RSEQ_SIG
 int
 sched_getcpu (void)
 {
   int cpu_id = THREAD_GETMEM_VOLATILE (THREAD_SELF, rseq_area.cpu_id);
   return __glibc_likely (cpu_id >= 0) ? cpu_id : vsyscall_sched_getcpu ();
 }
-#else /* RSEQ_SIG */
-int
-sched_getcpu (void)
-{
-  return vsyscall_sched_getcpu ();
-}
-#endif /* RSEQ_SIG */

base-commit: 2173173d57971d042c0ad4b281431ae127e9b5b8