Add the __libc_single_threaded variable

The variable is placed in libc.so, and it can be true only in
an outer libc, not libcs loaded via dlmopen or static dlopen.
Since thread creation from inner namespaces does not work,
pthread_create can update __libc_single_threaded directly.

Using __libc_early_init and its initial flag, implementation of this
variable is very straightforward.  A future version may reset the flag
during fork (but not in an inner namespace), or after joining all
threads except one.

Reviewed-by: DJ Delorie <dj@redhat.com>

NEWS

@@ -41,6 +41,12 @@ Major new features:
   The GNU C Library manual has details on integration of Restartable
   Sequences.
 
+* The GNU C Library now provides the header file <sys/single_threaded.h>
+  which declares the variable __libc_single_threaded.  Applications are
+  encouraged to use this variable for single-thread optimizations,
+  instead of weak references to symbols historically defined in
+  libpthread.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The deprecated <sys/sysctl.h> header and the sysctl function have been

elf/Makefile

@@ -155,7 +155,9 @@ endif
 tests-static-normal := tst-leaks1-static tst-array1-static tst-array5-static \
 	       tst-dl-iter-static \
 	       tst-tlsalign-static tst-tlsalign-extern-static \
-	       tst-linkall-static tst-env-setuid tst-env-setuid-tunables
+	       tst-linkall-static tst-env-setuid tst-env-setuid-tunables \
+	       tst-single_threaded-static tst-single_threaded-pthread-static
+
 tests-static-internal := tst-tls1-static tst-tls2-static \
 	       tst-ptrguard1-static tst-stackguard1-static \
 	       tst-tls1-static-non-pie tst-libc_dlvsym-static
@@ -174,9 +176,11 @@ tests-internal := tst-tls1 tst-tls2 $(tests-static-internal)
 tests-static := $(tests-static-normal) $(tests-static-internal)
 
 ifeq (yes,$(build-shared))
-tests-static += tst-tls9-static
-tst-tls9-static-ENV = \
-       LD_LIBRARY_PATH=$(objpfx):$(common-objpfx):$(common-objpfx)dlfcn
+tests-static += tst-tls9-static tst-single_threaded-static-dlopen
+static-dlopen-environment = \
+  LD_LIBRARY_PATH=$(objpfx):$(common-objpfx):$(common-objpfx)dlfcn
+tst-tls9-static-ENV = $(static-dlopen-environment)
+tst-single_threaded-static-dlopen-ENV = $(static-dlopen-environment)
 
 tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 constload1 order noload filter \
@@ -204,7 +208,8 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-dlopen-self tst-auditmany tst-initfinilazyfail tst-dlopenfail \
 	 tst-dlopenfail-2 \
 	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
-	 tst-audit14 tst-audit15 tst-audit16
+	 tst-audit14 tst-audit15 tst-audit16 \
+	 tst-single_threaded tst-single_threaded-pthread
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -317,7 +322,9 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-dlopenfailmod1 tst-dlopenfaillinkmod tst-dlopenfailmod2 \
 		tst-dlopenfailmod3 tst-ldconfig-ld-mod \
 		tst-filterobj-flt tst-filterobj-aux tst-filterobj-filtee \
-		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3
+		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3 \
+		tst-single_threaded-mod1 tst-single_threaded-mod2 \
+		tst-single_threaded-mod3 tst-single_threaded-mod4
 # Most modules build with _ISOMAC defined, but those filtered out
 # depend on internal headers.
 modules-names-tests = $(filter-out ifuncmod% tst-libc_dlvsym-dso tst-tlsmod%,\
@@ -1748,3 +1755,17 @@ $(objpfx)tst-auxobj: $(objpfx)tst-filterobj-aux.so
 $(objpfx)tst-auxobj-dlopen: $(libdl)
 $(objpfx)tst-auxobj.out: $(objpfx)tst-filterobj-filtee.so
 $(objpfx)tst-auxobj-dlopen.out: $(objpfx)tst-filterobj-filtee.so
+
+$(objpfx)tst-single_threaded: $(objpfx)tst-single_threaded-mod1.so $(libdl)
+$(objpfx)tst-single_threaded.out: \
+  $(objpfx)tst-single_threaded-mod2.so $(objpfx)tst-single_threaded-mod3.so
+$(objpfx)tst-single_threaded-static-dlopen: \
+  $(objpfx)tst-single_threaded-mod1.o $(common-objpfx)dlfcn/libdl.a
+$(objpfx)tst-single_threaded-static-dlopen.out: \
+  $(objpfx)tst-single_threaded-mod2.so
+$(objpfx)tst-single_threaded-pthread: \
+  $(objpfx)tst-single_threaded-mod1.so $(libdl) $(shared-thread-library)
+$(objpfx)tst-single_threaded-pthread.out: \
+  $(objpfx)tst-single_threaded-mod2.so $(objpfx)tst-single_threaded-mod3.so \
+  $(objpfx)tst-single_threaded-mod4.so
+$(objpfx)tst-single_threaded-pthread-static: $(static-thread-library)

elf/libc_early_init.c

@@ -19,13 +19,18 @@
 #include <ctype.h>
 #include <libc-early-init.h>
 #include <rseq-internal.h>
+#include <sys/single_threaded.h>
 
 void
 __libc_early_init (_Bool initial)
 {
   /* Initialize ctype data.  */
   __ctype_init ();
+
   /* Register rseq ABI to the kernel for the main program's libc.   */
   if (initial)
     rseq_register_current_thread ();
+
+  /* Only the outer namespace is marked as single-threaded.  */
+  __libc_single_threaded = initial;
 }

elf/tst-single_threaded-mod1.c

@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 1.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_1 (void)
+{
+  return __libc_single_threaded;
+}

elf/tst-single_threaded-mod2.c

@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 2.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_2 (void)
+{
+  return __libc_single_threaded;
+}

elf/tst-single_threaded-mod3.c

@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 3.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_3 (void)
+{
+  return __libc_single_threaded;
+}

elf/tst-single_threaded-mod4.c

@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 4.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_4 (void)
+{
+  return __libc_single_threaded;
+}

elf/tst-single_threaded-pthread-static.c

@@ -0,0 +1,86 @@
+/* Test support for single-thread optimizations.  With threads, static version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This test is a stripped-down version of
+   tst-single_threaded-pthread.c, without any loading of dynamic
+   objects.  */
+
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xthread.h>
+#include <sys/single_threaded.h>
+
+/* First barrier synchronizes main thread, thread 1, thread 2.  */
+static pthread_barrier_t barrier1;
+
+/* Second barrier synchronizes main thread, thread 2.  */
+static pthread_barrier_t barrier2;
+
+static void *
+threadfunc (void *closure)
+{
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Wait for the main thread and the other thread.  */
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Second thread waits on second barrier, too.  */
+  if (closure != NULL)
+    xpthread_barrier_wait (&barrier2);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+
+  /* Two threads plus main thread.  */
+  xpthread_barrier_init (&barrier1, NULL, 3);
+
+  /* Main thread and second thread.  */
+  xpthread_barrier_init (&barrier2, NULL, 2);
+
+  pthread_t thr1 = xpthread_create (NULL, threadfunc, NULL);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  pthread_t thr2 = xpthread_create (NULL, threadfunc, &thr2);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Join first thread.  This should not bring us back into
+     single-threaded mode.  */
+  xpthread_join (thr1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* We may be back in single-threaded mode after joining both
+     threads, but this is not guaranteed.  */
+  xpthread_barrier_wait (&barrier2);
+  xpthread_join (thr2);
+  printf ("info: __libc_single_threaded after joining all threads: %d\n",
+          __libc_single_threaded);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

elf/tst-single_threaded-pthread.c

@@ -0,0 +1,174 @@
+/* Test support for single-thread optimizations.  With threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+#include <sys/single_threaded.h>
+
+/* First barrier synchronizes main thread, thread 1, thread 2.  */
+static pthread_barrier_t barrier1;
+
+/* Second barrier synchronizes main thread, thread 2.  */
+static pthread_barrier_t barrier2;
+
+/* Defined in tst-single-threaded-mod1.so.  */
+_Bool single_threaded_1 (void);
+
+/* Initialized via dlsym.  */
+static _Bool (*single_threaded_2) (void);
+static _Bool (*single_threaded_3) (void);
+static _Bool (*single_threaded_4) (void);
+
+static void *
+threadfunc (void *closure)
+{
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+
+  /* Wait until the main thread loads more functions.  */
+  xpthread_barrier_wait (&barrier1);
+
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+
+  /* Second thread waits on second barrier, too.  */
+  if (closure != NULL)
+    xpthread_barrier_wait (&barrier2);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+
+  return NULL;
+}
+
+/* Used for closure arguments to the subprocess function.  */
+static char expected_false = 0;
+static char expected_true = 1;
+
+/* A subprocess inherits currently inherits the single-threaded state
+   of the parent process.  */
+static void
+subprocess (void *closure)
+{
+  const char *expected = closure;
+  TEST_COMPARE (__libc_single_threaded, *expected);
+  TEST_COMPARE (single_threaded_1 (), *expected);
+  if (single_threaded_2 != NULL)
+    TEST_COMPARE (single_threaded_2 (), *expected);
+  if (single_threaded_3 != NULL)
+    TEST_COMPARE (single_threaded_3 (), *expected);
+  if (single_threaded_4 != NULL)
+    TEST_VERIFY (!single_threaded_4 ());
+}
+
+static int
+do_test (void)
+{
+  printf ("info: main __libc_single_threaded address: %p\n",
+          &__libc_single_threaded);
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  support_isolate_in_subprocess (subprocess, &expected_true);
+
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  single_threaded_2 = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (single_threaded_2 ());
+
+  /* Two threads plus main thread.  */
+  xpthread_barrier_init (&barrier1, NULL, 3);
+
+  /* Main thread and second thread.  */
+  xpthread_barrier_init (&barrier2, NULL, 2);
+
+  pthread_t thr1 = xpthread_create (NULL, threadfunc, NULL);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  pthread_t thr2 = xpthread_create (NULL, threadfunc, &thr2);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Delayed library load, while already multi-threaded.  */
+  void *handle_mod3 = xdlopen ("tst-single_threaded-mod3.so", RTLD_LAZY);
+  single_threaded_3 = xdlsym (handle_mod3, "single_threaded_3");
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Same with dlmopen.  */
+  void *handle_mod4 = dlmopen (LM_ID_NEWLM, "tst-single_threaded-mod4.so",
+                               RTLD_LAZY);
+  single_threaded_4 = xdlsym (handle_mod4, "single_threaded_4");
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Run the newly loaded functions from the other threads as
+     well.  */
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Join first thread.  This should not bring us back into
+     single-threaded mode.  */
+  xpthread_join (thr1);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* We may be back in single-threaded mode after joining both
+     threads, but this is not guaranteed.  */
+  xpthread_barrier_wait (&barrier2);
+  xpthread_join (thr2);
+  printf ("info: __libc_single_threaded after joining all threads: %d\n",
+          __libc_single_threaded);
+
+  xdlclose (handle_mod4);
+  xdlclose (handle_mod3);
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

elf/tst-single_threaded-static-dlopen.c

@@ -0,0 +1,57 @@
+/* Test support for single-thread optimizations.  No threads, static dlopen.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* In a static dlopen scenario, the single-threaded optimization is
+   not possible because their is no globally shared dynamic linker
+   across all namespaces.  */
+
+#include <stddef.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+#include <sys/single_threaded.h>
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+
+  /* Defined in tst-single-threaded-mod1.o.  */
+  extern _Bool single_threaded_1 (void);
+  TEST_VERIFY (single_threaded_1 ());
+
+  /* A failed dlopen does not change the multi-threaded status.  */
+  TEST_VERIFY (dlopen ("tst-single_threaded-does-not-exist.so", RTLD_LAZY)
+               == NULL);
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+
+  /* And neither does a successful dlopen for outer (static) libc.  */
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  _Bool (*single_threaded_2) (void)
+    = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  /* The inner libc always assumes multi-threaded use.  */
+  TEST_VERIFY (!single_threaded_2 ());
+
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

elf/tst-single_threaded-static.c

@@ -0,0 +1,29 @@
+/* Test support for single-thread optimizations.  Static, no threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <sys/single_threaded.h>
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  return 0;
+}
+
+#include <support/test-driver.c>

elf/tst-single_threaded.c

@@ -0,0 +1,70 @@
+/* Test support for single-thread optimizations.  No threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/xdlfcn.h>
+#include <sys/single_threaded.h>
+
+/* Defined in tst-single-threaded-mod1.so.  */
+extern _Bool single_threaded_1 (void);
+
+/* Initialized via dlsym.  */
+_Bool (*single_threaded_2) (void);
+_Bool (*single_threaded_3) (void);
+
+static void
+subprocess (void *closure)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  if (single_threaded_2 != NULL)
+    TEST_VERIFY (single_threaded_2 ());
+  if (single_threaded_3 != NULL)
+    TEST_VERIFY (!single_threaded_3 ());
+}
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  single_threaded_2 = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  /* The current implementation treats the inner namespace as
+     multi-threaded.  */
+  void *handle_mod3 = dlmopen (LM_ID_NEWLM, "tst-single_threaded-mod3.so",
+                               RTLD_LAZY);
+  single_threaded_3 = xdlsym (handle_mod3, "single_threaded_3");
+  TEST_VERIFY (!single_threaded_3 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  xdlclose (handle_mod3);
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>

htl/pt-create.c

@@ -24,6 +24,7 @@
 
 #include <atomic.h>
 #include <hurd/resource.h>
+#include <sys/single_threaded.h>
 
 #include <pt-internal.h>
 #include <pthreadP.h>
@@ -104,6 +105,10 @@ __pthread_create_internal (struct __pthread **thread,
   sigset_t sigset;
   size_t stacksize;
 
+  /* Avoid a data race in the multi-threaded case.  */
+  if (__libc_single_threaded)
+    __libc_single_threaded = 0;
+
   /* Allocate a new thread structure.  */
   err = __pthread_alloc (&pthread);
   if (err)

include/sys/single_threaded.h

@@ -0,0 +1 @@
+#include <misc/sys/single_threaded.h>

misc/Makefile

@@ -37,7 +37,8 @@ headers	:= sys/uio.h bits/uio-ext.h bits/uio_lim.h \
 	   bits/syslog.h bits/syslog-ldbl.h bits/syslog-path.h bits/error.h \
 	   bits/select2.h bits/hwcap.h sys/auxv.h \
 	   sys/sysmacros.h bits/sysmacros.h bits/types/struct_iovec.h \
-	   bits/err-ldbl.h bits/error-ldbl.h
+	   bits/err-ldbl.h bits/error-ldbl.h \
+	   sys/single_threaded.h
 
 routines := brk sbrk sstk ioctl \
 	    readv writev preadv preadv64 pwritev pwritev64 \
@@ -72,7 +73,7 @@ routines := brk sbrk sstk ioctl \
 	    fgetxattr flistxattr fremovexattr fsetxattr getxattr \
 	    listxattr lgetxattr llistxattr lremovexattr lsetxattr \
 	    removexattr setxattr getauxval ifunc-impl-list makedev \
-	    allocate_once fd_to_filename
+	    allocate_once fd_to_filename single_threaded
 
 generated += tst-error1.mtrace tst-error1-mem.out \
   tst-allocate_once.mtrace tst-allocate_once-mem.out

misc/Versions

@@ -161,6 +161,9 @@ libc {
   GLIBC_2.30 {
     twalk_r;
   }
+  GLIBC_2.32 {
+    __libc_single_threaded;
+  }
   GLIBC_PRIVATE {
     __madvise;
     __mktemp;

misc/single_threaded.c

@@ -0,0 +1,27 @@
+/* Support for single-thread optimizations.  Statically linked version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+/* In dynamically linked programs, this variable is initialized in
+   __libc_early_init (as false for inner libcs).  */
+#ifdef SHARED
+char __libc_single_threaded;
+#else
+char __libc_single_threaded = 1;
+#endif

misc/sys/single_threaded.h

@@ -0,0 +1,33 @@
+/* Support for single-thread optimizations.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_SINGLE_THREADED_H
+#define _SYS_SINGLE_THREADED_H
+
+#include <features.h>
+
+__BEGIN_DECLS
+
+/* If this variable is non-zero, then the current thread is the only
+   thread in the process image.  If it is zero, the process might be
+   multi-threaded.  */
+extern char __libc_single_threaded;
+
+__END_DECLS
+
+#endif /* _SYS_SINGLE_THREADED_H */

nptl/pthread_create.c

@@ -35,6 +35,7 @@
 #include <tls-setup.h>
 #include <rseq-internal.h>
 #include "libioP.h"
+#include <sys/single_threaded.h>
 
 #include <shlib-compat.h>
 
@@ -625,6 +626,10 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
 {
   STACK_VARIABLES;
 
+  /* Avoid a data race in the multi-threaded case.  */
+  if (__libc_single_threaded)
+    __libc_single_threaded = 0;
+
   const struct pthread_attr *iattr = (struct pthread_attr *) attr;
   union pthread_attr_transparent default_attr;
   bool destroy_default_attr = false;

sysdeps/generic/libc.abilist

@@ -0,0 +1 @@
+GLIBC_2.32 __libc_single_threaded D 0x1

sysdeps/mach/hurd/i386/libc.abilist

@@ -2181,6 +2181,7 @@ GLIBC_2.3.4 setsourcefilter F
 GLIBC_2.3.4 xdr_quad_t F
 GLIBC_2.3.4 xdr_u_quad_t F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 mach_print F
 GLIBC_2.32 mremap F
 GLIBC_2.32 thrd_current F

sysdeps/unix/sysv/linux/aarch64/libc.abilist

@@ -2149,6 +2149,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/alpha/libc.abilist

@@ -2231,6 +2231,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/arm/be/libc.abilist

@@ -133,6 +133,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/arm/le/libc.abilist

@@ -130,6 +130,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/csky/libc.abilist

@@ -2093,6 +2093,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/hppa/libc.abilist

@@ -2052,6 +2052,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/i386/libc.abilist

@@ -2218,6 +2218,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/ia64/libc.abilist

@@ -2084,6 +2084,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist

@@ -134,6 +134,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist

@@ -2164,6 +2164,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/microblaze/be/libc.abilist

@@ -2144,6 +2144,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/microblaze/le/libc.abilist

@@ -2141,6 +2141,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist

@@ -2135,6 +2135,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist

@@ -2133,6 +2133,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist

@@ -2141,6 +2141,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist

@@ -2135,6 +2135,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/nios2/libc.abilist

@@ -2182,6 +2182,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist

@@ -2191,6 +2191,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist

@@ -2224,6 +2224,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist

@@ -2054,6 +2054,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist

@@ -2279,6 +2279,7 @@ GLIBC_2.32 __isoc99_vsscanfieee128 F
 GLIBC_2.32 __isoc99_vswscanfieee128 F
 GLIBC_2.32 __isoc99_vwscanfieee128 F
 GLIBC_2.32 __isoc99_wscanfieee128 F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __obstack_printf_chkieee128 F
 GLIBC_2.32 __obstack_printfieee128 F
 GLIBC_2.32 __obstack_vprintf_chkieee128 F

sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist

@@ -2111,6 +2111,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist

@@ -2189,6 +2189,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist

@@ -2090,6 +2090,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/sh/be/libc.abilist

@@ -2059,6 +2059,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/sh/le/libc.abilist

@@ -2056,6 +2056,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist

@@ -2180,6 +2180,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist

@@ -2107,6 +2107,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/x86_64/64/libc.abilist

@@ -2065,6 +2065,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F

sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist

@@ -2162,6 +2162,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F