hurd: Only check for TLS initialization inside rtld or in static builds

When glibc is built as a shared library, TLS is always initialized by
the call of TLS_INIT_TP () macro made inside the dynamic loader, prior
to running the main program (see dl-call_tls_init_tp.h). We can take
advantage of this: we know for sure that __LIBC_NO_TLS () will evaluate
to 0 in all other cases, so let the compiler know that explicitly too.

Also, only define _hurd_tls_init () and TLS_INIT_TP () under the same
conditions (either !SHARED or inside rtld), to statically assert that
this is the case.

Other than a microoptimization, this also helps with avoiding awkward
sharing of the __libc_tls_initialized variable between ld.so and libc.so
that we would have to do otherwise -- we know for sure that no sharing
is required, simply because __libc_tls_initialized would always be set
to true inside libc.so.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230319151017.531737-25-bugaevc@gmail.com>

1 files changed, 12 insertions, 7 deletions

diff --git a/sysdeps/mach/hurd/x86_64/tls.h b/sysdeps/mach/hurd/x86_64/tls.h
index cf74e1f441..da504d9c81 100644
--- a/sysdeps/mach/hurd/x86_64/tls.h
+++ b/sysdeps/mach/hurd/x86_64/tls.h
@@ -68,10 +68,6 @@ _Static_assert (offsetof (tcbhead_t, stack_guard) == 0x28,
 _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x70,
                 "split stack pointer offset");
 
-extern unsigned char __libc_tls_initialized;
-
-# define __LIBC_NO_TLS() __builtin_expect (!__libc_tls_initialized, 0)
-
 /* The TCB can have any size and the memory following the address the
    thread pointer points to is unspecified.  Allocate the TCB there.  */
 # define TLS_TCB_AT_TP	1
@@ -87,8 +83,6 @@ extern unsigned char __libc_tls_initialized;
 # define TCB_ALIGNMENT	64
 
 
-# define TLS_INIT_TP(descr) _hurd_tls_init ((tcbhead_t *) (descr))
-
 # define THREAD_SELF							\
   (*(tcbhead_t * __seg_fs *) offsetof (tcbhead_t, tcb))
 /* Read member of the thread descriptor directly.  */
@@ -174,6 +168,10 @@ _hurd_tls_new (thread_t child, tcbhead_t *tcb)
                              i386_FSGS_BASE_STATE_COUNT);
 }
 
+# if !defined (SHARED) || IS_IN (rtld)
+extern unsigned char __libc_tls_initialized;
+#  define __LIBC_NO_TLS() __builtin_expect (!__libc_tls_initialized, 0)
+
 static inline bool __attribute__ ((unused))
 _hurd_tls_init (tcbhead_t *tcb)
 {
@@ -184,11 +182,18 @@ _hurd_tls_init (tcbhead_t *tcb)
   tcb->multiple_threads = 1;
 
   err = _hurd_tls_new (self, tcb);
+  if (err == 0)
+    __libc_tls_initialized = 1;
   __mach_port_deallocate (__mach_task_self (), self);
-  __libc_tls_initialized = 1;
   return err == 0;
 }
 
+#  define TLS_INIT_TP(descr) _hurd_tls_init ((tcbhead_t *) (descr))
+# else /* defined (SHARED) && !IS_IN (rtld) */
+#  define __LIBC_NO_TLS() 0
+# endif
+
+
 
 /* Global scope switch support.  */
 # define THREAD_GSCOPE_FLAG_UNUSED 0