[PATCH 16/16] linux: Optimize fallback 32-bit clock_getres

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

This patch avoid probing the __NR_clock_getttime64 syscall each time
__clock_gettime64 is issued on a kernel without 64 bit time support.
Once ENOSYS is obtained, only 32-bit clock_gettime are used.

The following snippet:

  clock_getres (CLOCK_REALTIME, &(struct timespec) { 0 });
  clock_getres (CLOCK_MONOTONIC, &(struct timespec) { 0 });
  clock_getres (CLOCK_BOOTTIME, &(struct timespec) { 0 });
  clock_getres (20, &(struct timespec) { 0 });

On a kernel without 64 bit time support issues the syscalls:

  syscall_0x196(0, 0xffb83330, [...]) = -1 ENOSYS (Function not implemented)
  clock_getres(CLOCK_REALTIME, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_MONOTONIC, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_BOOTTIME, {tv_sec=0, tv_nsec=1}) = 0

Checked on i686-linux-gnu on 4.15 kernel.
---
 sysdeps/unix/sysv/linux/clock_getres.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/clock_getres.c b/sysdeps/unix/sysv/linux/clock_getres.c
index 3d59f11d71..c1e1cf60c6 100644
--- a/sysdeps/unix/sysv/linux/clock_getres.c
+++ b/sysdeps/unix/sysv/linux/clock_getres.c
@@ -42,13 +42,21 @@ __clock_getres64 (clockid_t clock_id, struct __timespec64 *res)
   int r;
   /* Old 32-bit ABI with possible 64-bit time_t support.  */
 # ifdef __NR_clock_getres_time64
+  /* Avoid issue a __NR_clock_getres_time64 syscall on kernels that do not
+     support 64-bit time_t.  */
+  static int time64_support = 1;
+  if (atomic_load_relaxed (&time64_support) == 1)
+    {
 #  ifdef HAVE_CLOCK_GETRES64_VSYSCALL
-  r = INLINE_VSYSCALL (clock_getres_time64, 2, clock_id, res);
+      r = INLINE_VSYSCALL (clock_getres_time64, 2, clock_id, res);
 #  else
-  r = INLINE_SYSCALL_CALL (clock_getres_time64, clock_id, res);
+      r = INLINE_SYSCALL_CALL (clock_getres_time64, clock_id, res);
 #  endif
-  if (r == 0 || errno != ENOSYS)
-    return r;
+      if (r == 0 || errno != ENOSYS)
+	return r;
+
+      atomic_store_relaxed (&time64_support, 0);
+    }
 # endif
   /* Fallback code that uses 32-bit support.  */
   struct timespec ts32;
-- 
2.17.1