Re: [PATCH v2 2/2] sysdeps/clock_gettime: Use clock_gettime64 if avaliable

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

On 09/11/2019 23:56, Alistair Francis wrote:
> With the clock_gettime64 call we prefer to use vDSO. There is no call
> to clock_gettime64 on glibc with older headers and kernel 5.1+ if it
> doesn't support vDSO.
> ---
> This was patch was runtime tested with RV32 and RV64
> It was build tested using the ./scripts/build-many-glibcs.py script.
> 
> I also ran:
> $ make ; make install ; make check
> tests on native ARM (32-bit) with the following three confiugrations:
>  - 4.19 Kernel and 4.19 Headers
>  - 5.2 Kernel and 4.19 Headers
>  - 5.2 Kernel and 5.2 Headers
> and didn't see any regressions
> 
> v2:
>  - Add commit message
>  - Change ret_64 to int
> 
>  include/time.h                          |  3 ++
>  sysdeps/unix/sysv/linux/clock_gettime.c | 50 ++++++++++++++++++++++++-
>  2 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/include/time.h b/include/time.h
> index d7800eb30f8..c19c73ae09f 100644
> --- a/include/time.h
> +++ b/include/time.h
> @@ -211,11 +211,14 @@ extern double __difftime (time_t time1, time_t time0);
>  
>  #if __TIMESIZE == 64
>  # define __clock_nanosleep_time64 __clock_nanosleep
> +# define __clock_gettime64 __clock_gettime
>  #else
>  extern int __clock_nanosleep_time64 (clockid_t clock_id,
>                                       int flags, const struct __timespec64 *req,
>                                       struct __timespec64 *rem);
>  libc_hidden_proto (__clock_nanosleep_time64)
> +extern int __clock_gettime64 (clockid_t clock_id, struct __timespec64 *tp);
> +libc_hidden_proto (__clock_gettime64)
>  #endif
>  
>  /* Use in the clock_* functions.  Size of the field representing the
> diff --git a/sysdeps/unix/sysv/linux/clock_gettime.c b/sysdeps/unix/sysv/linux/clock_gettime.c
> index ca40983f6ca..e7af9902f3e 100644
> --- a/sysdeps/unix/sysv/linux/clock_gettime.c
> +++ b/sysdeps/unix/sysv/linux/clock_gettime.c
> @@ -17,6 +17,7 @@
>     <https://www.gnu.org/licenses/>.  */
>  
>  #include <sysdep.h>
> +#include <kernel-features.h>
>  #include <errno.h>
>  #include <time.h>
>  #include "kernel-posix-cpu-timers.h"

It requires to define HAVE_SYSCALL if HAVE_CLOCK_GETTIME64_VSYSCALL is also
defined, otherwise architectures that define __ASSUME_TIME64_SYSCALLS and
HAVE_CLOCK_GETTIME64_VSYSCALL will not use the vdso.

> @@ -30,10 +31,57 @@
>  
>  /* Get current value of CLOCK and store it in TP.  */
>  int
> +__clock_gettime64 (clockid_t clock_id, struct __timespec64 *tp)
> +{
> +#ifdef __ASSUME_TIME64_SYSCALLS
> +# ifndef __NR_clock_gettime64
> +#  define __NR_clock_gettime64 __NR_clock_gettime
> +#  define __vdso_clock_gettime64 __vdso_clock_gettime
> +# endif
> +   return INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);

Right, so it would call either __vdso_clock_gettime/__NR_clock_gettime or
__vdso_clock_gettime64/__NR_clock_gettime64.

> +#else
> +# if defined __NR_clock_gettime64 && defined HAVE_CLOCK_GETTIME64_VSYSCALL

I think we can assume that if the ABI provides the time64 vDSO symbol
it also provides the syscall (__NR_clock_gettime64).  So we can just 
check if HAVE_CLOCK_GETTIME64_VSYSCALL is defined. Something like:

#ifdef __ASSUME_TIME64_SYSCALLS
# ifndef __NR_clock_gettime64
#  define __NR_clock_gettime64   __NR_clock_gettime
#  define __vdso_clock_gettime64 __vdso_clock_gettime
# endif
   return INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
#else
# if defined HAVE_CLOCK_GETTIME64_VSYSCALL
  int ret64 = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
  if (ret64 == 0 || errno != ENOSYS)
    return ret64;
# endif
  struct timespec tp32;
  int ret = INLINE_VSYSCALL (clock_gettime, 2, clock_id, &tp32);
  if (ret == 0)
    *tp = valid_timespec_to_timespec64 (tp32);
  return ret;
#endif


> +  int ret_64;
> +  ret_64 = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
> +
> +  if (ret_64 == 0 || errno != ENOSYS)
> +    return ret_64;
> +# endif /* __NR_clock_gettime64 && HAVE_CLOCK_GETTIME64_VSYSCALL  */
> +  int ret;
> +  struct timespec tp32;
> +
> +  ret = INLINE_VSYSCALL (clock_gettime, 2, clock_id, &tp32);
> +
> +  if (ret == 0)
> +    *tp = valid_timespec_to_timespec64 (tp32);
> +
> +  return ret;
> +#endif /* __ASSUME_TIME64_SYSCALLS  */

Ok.

> +}
> +
> +#if __TIMESIZE != 64
> +int
>  __clock_gettime (clockid_t clock_id, struct timespec *tp)
>  {
> -  return INLINE_VSYSCALL (clock_gettime, 2, clock_id, tp);
> +  int ret;
> +  struct __timespec64 tp64;
> +
> +  ret = __clock_gettime64 (clock_id, &tp64);
> +
> +  if (ret == 0)
> +    {
> +      if (! in_time_t_range (tp64.tv_sec))
> +        {
> +          __set_errno (EOVERFLOW);
> +          return -1;
> +        }
> +
> +      *tp = valid_timespec64_to_timespec (tp64);
> +    }
> +
> +  return ret;
>  }
> +#endif
>  libc_hidden_def (__clock_gettime)
>  
>  versioned_symbol (libc, __clock_gettime, clock_gettime, GLIBC_2_17);

Ok.