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>

1 files changed, 5 insertions, 0 deletions

diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index f348a6f6dd..e05013e317 100644
--- a/nptl/pthread_create.c
+++ b/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;