Linux: optimize clone3 internal usage

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.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

diff --git a/include/clone_internal.h b/include/clone_internal.h
index 3b6cd85f026fa13c3e2b2dd45ec1adac78653330..dd380f119e174c0b3ca47c81239928a428f0a4bb 100644 (file)
--- a/include/clone_internal.h
+++ b/include/clone_internal.h
@@ -24,6 +24,11 @@ extern int __clone3 (struct clone_args *__cl_args, size_t __size,
    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-issuing the syscall if
+   it fails with ENOSYS.  */
+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 e125c9125ed707fff4b65a483ccc401302f832e1..790739cfceb4b04728cc3312952be74efc795e2e 100644 (file)
--- 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 a23d0079a13c3ecd66b09c1fd9515de323f52628..4b5c4afbc141a6a18956cac06618acc3c99afe0f 100644 (file)
--- a/sysdeps/unix/sysv/linux/kernel-features.h
+++ b/sysdeps/unix/sysv/linux/kernel-features.h
@@ -241,4 +241,13 @@
 # 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 */