gdb/gdbsupport/thread-pool.c

   1 /* Thread pool
   2
   3    Copyright (C) 2019 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #include "common-defs.h"
  21
  22 #if CXX_STD_THREAD
  23
  24 #include "gdbsupport/thread-pool.h"
  25 #include "gdbsupport/alt-stack.h"
  26 #include "gdbsupport/block-signals.h"
  27 #include <algorithm>
  28
  29 /* On the off chance that we have the pthread library on a Windows
  30    host, but std::thread is not using it, avoid calling
  31    pthread_setname_np on Windows.  */
  32 #ifndef _WIN32
  33 #ifdef HAVE_PTHREAD_SETNAME_NP
  34 #define USE_PTHREAD_SETNAME_NP
  35 #endif
  36 #endif
  37
  38 #ifdef USE_PTHREAD_SETNAME_NP
  39
  40 #include <pthread.h>
  41
  42 /* Handle platform discrepancies in pthread_setname_np: macOS uses a
  43    single-argument form, while Linux uses a two-argument form.  This
  44    wrapper handles the difference.  */
  45
  46 ATTRIBUTE_UNUSED static void
  47 set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
  48 {
  49   set_name (pthread_self (), name);
  50 }
  51
  52 ATTRIBUTE_UNUSED static void
  53 set_thread_name (void (*set_name) (const char *), const char *name)
  54 {
  55   set_name (name);
  56 }
  57
  58 #endif  /* USE_PTHREAD_SETNAME_NP */
  59
  60 namespace gdb
  61 {
  62
  63 /* The thread pool detach()s its threads, so that the threads will not
  64    prevent the process from exiting.  However, it was discovered that
  65    if any detached threads were still waiting on a condition variable,
  66    then the condition variable's destructor would wait for the threads
  67    to exit -- defeating the purpose.
  68
  69    Allocating the thread pool on the heap and simply "leaking" it
  70    avoids this problem.
  71 */
  72 thread_pool *thread_pool::g_thread_pool = new thread_pool ();
  73
  74 thread_pool::~thread_pool ()
  75 {
  76   /* Because this is a singleton, we don't need to clean up.  The
  77      threads are detached so that they won't prevent process exit.
  78      And, cleaning up here would be actively harmful in at least one
  79      case -- see the comment by the definition of g_thread_pool.  */
  80 }
  81
  82 void
  83 thread_pool::set_thread_count (size_t num_threads)
  84 {
  85   std::lock_guard<std::mutex> guard (m_tasks_mutex);
  86
  87   /* If the new size is larger, start some new threads.  */
  88   if (m_thread_count < num_threads)
  89     {
  90       /* Ensure that signals used by gdb are blocked in the new
  91          threads.  */
  92       block_signals blocker;
  93       for (size_t i = m_thread_count; i < num_threads; ++i)
  94         {
  95           std::thread thread (&thread_pool::thread_function, this);
  96           thread.detach ();
  97         }
  98     }
  99   /* If the new size is smaller, terminate some existing threads.  */
 100   if (num_threads < m_thread_count)
 101     {
 102       for (size_t i = num_threads; i < m_thread_count; ++i)
 103         m_tasks.emplace ();
 104       m_tasks_cv.notify_all ();
 105     }
 106
 107   m_thread_count = num_threads;
 108 }
 109
 110 std::future<void>
 111 thread_pool::post_task (std::function<void ()> func)
 112 {
 113   std::packaged_task<void ()> t (func);
 114   std::future<void> f = t.get_future ();
 115
 116   if (m_thread_count == 0)
 117     {
 118       /* Just execute it now.  */
 119       t ();
 120     }
 121   else
 122     {
 123       std::lock_guard<std::mutex> guard (m_tasks_mutex);
 124       m_tasks.emplace (std::move (t));
 125       m_tasks_cv.notify_one ();
 126     }
 127   return f;
 128 }
 129
 130 void
 131 thread_pool::thread_function ()
 132 {
 133 #ifdef USE_PTHREAD_SETNAME_NP
 134   /* This must be done here, because on macOS one can only set the
 135      name of the current thread.  */
 136   set_thread_name (pthread_setname_np, "gdb worker");
 137 #endif
 138
 139   /* Ensure that SIGSEGV is delivered to an alternate signal
 140      stack.  */
 141   gdb::alternate_signal_stack signal_stack;
 142
 143   while (true)
 144     {
 145       optional<task> t;
 146
 147       {
 148         /* We want to hold the lock while examining the task list, but
 149            not while invoking the task function.  */
 150         std::unique_lock<std::mutex> guard (m_tasks_mutex);
 151         while (m_tasks.empty ())
 152           m_tasks_cv.wait (guard);
 153         t = std::move (m_tasks.front());
 154         m_tasks.pop ();
 155       }
 156
 157       if (!t.has_value ())
 158         break;
 159       (*t) ();
 160     }
 161 }
 162
 163 }
 164
 165 #endif /* CXX_STD_THREAD */