[PATCH v2 9/9] Linux: optimize clone3 internal usage

[Carlos O'Donell]

On 9/30/22 15:26, Adhemerval Zanella via Libc-alpha wrote:
> Now that clone3 is used on more architectures, add an optimization
> to avoid calling when glibc detects that it is no supported by the
> kernel.  It also adds __ASSUME_CLONE3, which allows skip this
> optimization and issue clone3 syscall directly.

Please post v3 with changes.

Suggest:

Add an optimization to avoid calling clone3 when glibc detects that there is
no kernel support.  It also adds __ASSUME_CLONE3, which allows skipping this
optimization and issuing the clone3 syscall directly.

> 
> It does not handle the the small window between 5.3 and 5.5 for
> posix_spawn (CLONE_CLEAR_SIGHAND was added in 5.5).
> 
> Checked on x86_64-linux-gnu.
> ---
>  include/clone_internal.h                  |  5 +++++
>  sysdeps/unix/sysv/linux/clone-internal.c  | 24 ++++++++++++++++++++++-
>  sysdeps/unix/sysv/linux/kernel-features.h |  9 +++++++++
>  3 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/include/clone_internal.h b/include/clone_internal.h
> index 320640e64b..385f3c5589 100644
> --- a/include/clone_internal.h
> +++ b/include/clone_internal.h
> @@ -7,6 +7,11 @@ extern __typeof (clone3) __clone3;
>     -1 with ENOSYS, fall back to clone or clone2.  */
>  extern int __clone_internal (struct clone_args *__cl_args,
>  			     int (*__func) (void *__arg), void *__arg);
> +/* clone3 wrapper with a sticky check to avoid re-issue the syscall if
> +   it fails with ENOSYS.  */

s/re-issue/re-issuing/g

> +extern int __clone3_internal (struct clone_args *cl_args,
> +			      int (*func) (void *args), void *arg)
> +     attribute_hidden;
>  /* The fallback code which calls clone/clone2 based on clone3 arguments.  */
>  extern int __clone_internal_fallback (struct clone_args *__cl_args,
>  				      int (*__func) (void *__arg),
> diff --git a/sysdeps/unix/sysv/linux/clone-internal.c b/sysdeps/unix/sysv/linux/clone-internal.c
> index 7bc991e033..39d76733db 100644
> --- a/sysdeps/unix/sysv/linux/clone-internal.c
> +++ b/sysdeps/unix/sysv/linux/clone-internal.c
> @@ -76,6 +76,28 @@ __clone_internal_fallback (struct clone_args *cl_args,
>    return ret;
>  }
>  
> +int
> +__clone3_internal (struct clone_args *cl_args, int (*func) (void *args),
> +		   void *arg)
> +{
> +#ifdef HAVE_CLONE3_WRAPPER
> +# if __ASSUME_CLONE3
> +  return __clone3 (cl_args, sizeof (*cl_args), func, arg);
> +# else
> +  static int clone3_supported = 1;
> +  if (atomic_load_relaxed (&clone3_supported) == 1)
> +    {
> +      int ret = __clone3 (cl_args, sizeof (*cl_args), func, arg);
> +      if (ret != -1 || errno != ENOSYS)
> +	return ret;
> +
> +      atomic_store_relaxed (&clone3_supported, 0);
> +    }
> +# endif
> +#endif
> +  __set_errno (ENOSYS);
> +  return -1;
> +}
>  
>  int
>  __clone_internal (struct clone_args *cl_args,
> @@ -83,7 +105,7 @@ __clone_internal (struct clone_args *cl_args,
>  {
>  #ifdef HAVE_CLONE3_WRAPPER
>    int saved_errno = errno;
> -  int ret = __clone3 (cl_args, sizeof (*cl_args), func, arg);
> +  int ret = __clone3_internal (cl_args, func, arg);
>    if (ret != -1 || errno != ENOSYS)
>      return ret;
>  
> diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h
> index 74adc3956b..4ecd08a98f 100644
> --- a/sysdeps/unix/sysv/linux/kernel-features.h
> +++ b/sysdeps/unix/sysv/linux/kernel-features.h
> @@ -236,4 +236,13 @@
>  # define __ASSUME_FUTEX_LOCK_PI2 0
>  #endif
>  
> +/* The clone3 system call was introduced across on most architectures in
> +   Linux 5.3.  Not all ports implements it, so it should be used along
> +   HAVE_CLONE3_WRAPPER define.  */
> +#if __LINUX_KERNEL_VERSION >= 0x050300
> +# define __ASSUME_CLONE3 1
> +#else
> +# define __ASSUME_CLONE3 0
> +#endif
> +
>  #endif /* kernel-features.h */

-- 
Cheers,
Carlos.