htl: Reimplement GSCOPE

This is a new implementation of GSCOPE which largely mirrors its NPTL
counterpart. Same as in NPTL, instead of a global flag shared between
threads, there is now a per-thread GSCOPE flag stored in each thread's
TCB. This makes entering and exiting a GSCOPE faster at the expense of
making THREAD_GSCOPE_WAIT () slower.

The largest win is the elimination of many redundant gsync_wake () RPC
calls; previously, even simplest programs would make dozens of fully
redundant gsync_wake () calls.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20210915171110.226187-3-bugaevc@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

1 files changed, 55 insertions, 0 deletions

diff --git a/sysdeps/htl/dl-thread_gscope_wait.c b/sysdeps/htl/dl-thread_gscope_wait.c
new file mode 100644
index 0000000000..7557e77aab
--- /dev/null
+++ b/sysdeps/htl/dl-thread_gscope_wait.c
@@ -0,0 +1,55 @@
+/* Out-of-line notification function for the GSCOPE locking mechanism.
+   Copyright (C) 2007-2021 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 <ldsodefs.h>
+#include <pthread.h>
+#include <htl/pt-internal.h>
+
+void
+__thread_gscope_wait (void)
+{
+  size_t i;
+  struct __pthread *t;
+  int *gscope_flagp;
+
+  __libc_rwlock_rdlock (GL (dl_pthread_threads_lock));
+
+  /* Iterate over the list of threads.  */
+  for (i = 0; i < GL (dl_pthread_num_threads); ++i)
+    {
+      t = GL (dl_pthread_threads[i]);
+      if (t == NULL || t->tcb->gscope_flag == THREAD_GSCOPE_FLAG_UNUSED)
+        continue;
+
+      gscope_flagp = &t->tcb->gscope_flag;
+
+      /* We have to wait until this thread is done with the global
+         scope.  First tell the thread that we are waiting and
+         possibly have to be woken.  */
+      if (atomic_compare_and_exchange_bool_acq (gscope_flagp,
+                                                THREAD_GSCOPE_FLAG_WAIT,
+                                                THREAD_GSCOPE_FLAG_USED))
+        continue;
+
+      do
+        lll_wait (gscope_flagp, THREAD_GSCOPE_FLAG_WAIT, LLL_PRIVATE);
+      while (*gscope_flagp == THREAD_GSCOPE_FLAG_WAIT);
+    }
+
+  __libc_rwlock_unlock (GL (dl_pthread_threads_lock));
+}