diff --git a/lto-plugin/config.h.in b/lto-plugin/config.h.in
index ddbde7619d5..029e782f1ee 100644
--- a/lto-plugin/config.h.in
+++ b/lto-plugin/config.h.in
@@ -9,6 +9,9 @@
 /* Define to 1 if you have the <memory.h> header file. */
 #undef HAVE_MEMORY_H
 
+/* Define to 1 if pthread.h is present. */
+#undef HAVE_PTHREAD_H
+
 /* Define to 1 if you have the <stdint.h> header file. */
 #undef HAVE_STDINT_H
 
diff --git a/lto-plugin/configure b/lto-plugin/configure
index b820accfd65..7d2f1ad308c 100755
--- a/lto-plugin/configure
+++ b/lto-plugin/configure
@@ -5643,6 +5643,9 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
+# The plug-in depends on pthreads
+LDFLAGS="-pthread"
+
 # Check whether -static-libgcc is supported.
 saved_LDFLAGS="$LDFLAGS"
 LDFLAGS="$LDFLAGS -static-libgcc"
@@ -6010,6 +6013,16 @@ else
 fi
 
 
+# Check for thread headers.
+ac_fn_c_check_header_mongrel "$LINENO" "pthread.h" "ac_cv_header_pthread_h" "$ac_includes_default"
+if test "x$ac_cv_header_pthread_h" = xyes; then :
+
+$as_echo "#define HAVE_PTHREAD_H 1" >>confdefs.h
+
+fi
+
+
+
 case `pwd` in
   *\ * | *\	*)
     { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: Libtool does not cope well with whitespace in \`pwd\`" >&5
@@ -12081,7 +12094,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 12084 "configure"
+#line 12097 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -12187,7 +12200,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 12190 "configure"
+#line 12203 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
diff --git a/lto-plugin/configure.ac b/lto-plugin/configure.ac
index bc5b618a495..75cf46ac5c7 100644
--- a/lto-plugin/configure.ac
+++ b/lto-plugin/configure.ac
@@ -13,6 +13,9 @@ AC_PROG_CC
 AC_SYS_LARGEFILE
 ACX_PROG_CC_WARNING_OPTS([-Wall], [ac_lto_plugin_warn_cflags])
 
+# The plug-in depends on pthreads
+LDFLAGS="-pthread"
+
 # Check whether -static-libgcc is supported.
 saved_LDFLAGS="$LDFLAGS"
 LDFLAGS="$LDFLAGS -static-libgcc"
@@ -87,6 +90,10 @@ AM_CONDITIONAL(LTO_PLUGIN_USE_SYMVER, [test "x$lto_plugin_use_symver" != xno])
 AM_CONDITIONAL(LTO_PLUGIN_USE_SYMVER_GNU, [test "x$lto_plugin_use_symver" = xgnu])
 AM_CONDITIONAL(LTO_PLUGIN_USE_SYMVER_SUN, [test "x$lto_plugin_use_symver" = xsun])
 
+# Check for thread headers.
+AC_CHECK_HEADER(pthread.h,
+  [AC_DEFINE(HAVE_PTHREAD_H, 1, [Define to 1 if pthread.h is present.])])
+
 AM_PROG_LIBTOOL
 ACX_LT_HOST_FLAGS
 AC_SUBST(target_noncanonical)
diff --git a/lto-plugin/lto-plugin.c b/lto-plugin/lto-plugin.c
index 00b760636dc..635e126946b 100644
--- a/lto-plugin/lto-plugin.c
+++ b/lto-plugin/lto-plugin.c
@@ -40,7 +40,11 @@ along with this program; see the file COPYING3.  If not see
 
 #ifdef HAVE_CONFIG_H
 #include "config.h"
+#if !HAVE_PTHREAD_H
+#error POSIX threads are mandatory dependency
 #endif
+#endif
+
 #if HAVE_STDINT_H
 #include <stdint.h>
 #endif
@@ -55,6 +59,7 @@ along with this program; see the file COPYING3.  If not see
 #include <unistd.h>
 #include <fcntl.h>
 #include <sys/types.h>
+#include <pthread.h>
 #ifdef HAVE_SYS_WAIT_H
 #include <sys/wait.h>
 #endif
@@ -157,6 +162,9 @@ enum symbol_style
   ss_uscore,	/* Underscore prefix all symbols.  */
 };
 
+/* Plug-in mutex.  */
+static pthread_mutex_t plugin_lock;
+
 static char *arguments_file_name;
 static ld_plugin_register_claim_file register_claim_file;
 static ld_plugin_register_all_symbols_read register_all_symbols_read;
@@ -1262,15 +1270,18 @@ claim_file_handler (const struct ld_plugin_input_file *file, int *claimed)
 			      lto_file.symtab.syms);
       check (status == LDPS_OK, LDPL_FATAL, "could not add symbols");
 
+      pthread_mutex_lock (&plugin_lock);
       num_claimed_files++;
       claimed_files =
 	xrealloc (claimed_files,
 		  num_claimed_files * sizeof (struct plugin_file_info));
       claimed_files[num_claimed_files - 1] = lto_file;
+      pthread_mutex_unlock (&plugin_lock);
 
       *claimed = 1;
     }
 
+  pthread_mutex_lock (&plugin_lock);
   if (offload_files == NULL)
     {
       /* Add dummy item to the start of the list.  */
@@ -1333,11 +1344,14 @@ claim_file_handler (const struct ld_plugin_input_file *file, int *claimed)
 	offload_files_last_lto = ofld;
       num_offload_files++;
     }
+  pthread_mutex_unlock (&plugin_lock);
 
   goto cleanup;
 
  err:
+  pthread_mutex_lock (&plugin_lock);
   non_claimed_files++;
+  pthread_mutex_unlock (&plugin_lock);
   free (lto_file.name);
 
  cleanup:
@@ -1415,6 +1429,12 @@ onload (struct ld_plugin_tv *tv)
   struct ld_plugin_tv *p;
   enum ld_plugin_status status;
 
+  if (pthread_mutex_init (&plugin_lock, NULL) != 0)
+    {
+      fprintf (stderr, "mutex init failed\n");
+      abort ();
+    }
+
   p = tv;
   while (p->tv_tag)
     {