gdbsupport/thread-pool.cc

   1 /* Thread pool
   2
   3    Copyright (C) 2019-2021 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 #include <system_error>
  29
  30 /* On the off chance that we have the pthread library on a Windows
  31    host, but std::thread is not using it, avoid calling
  32    pthread_setname_np on Windows.  */
  33 #ifndef _WIN32
  34 #ifdef HAVE_PTHREAD_SETNAME_NP
  35 #define USE_PTHREAD_SETNAME_NP
  36 #endif
  37 #endif
  38
  39 #ifdef USE_PTHREAD_SETNAME_NP
  40
  41 #include <pthread.h>
  42
  43 /* Handle platform discrepancies in pthread_setname_np: macOS uses a
  44    single-argument form, while Linux uses a two-argument form.  NetBSD
  45    takes a printf-style format and an argument.  This wrapper handles the
  46    difference.  */
  47
  48 ATTRIBUTE_UNUSED static void
  49 set_thread_name (int (*set_name) (pthread_t, const char *, void *),
  50                                   const char *name)
  51 {
  52   set_name (pthread_self (), "%s", const_cast<char *> (name));
  53 }
  54
  55 ATTRIBUTE_UNUSED static void
  56 set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
  57 {
  58   set_name (pthread_self (), name);
  59 }
  60
  61 /* The macOS man page says that pthread_setname_np returns "void", but
  62    the headers actually declare it returning "int".  */
  63 ATTRIBUTE_UNUSED static void
  64 set_thread_name (int (*set_name) (const char *), const char *name)
  65 {
  66   set_name (name);
  67 }
  68
  69 #endif  /* USE_PTHREAD_SETNAME_NP */
  70
  71 namespace gdb
  72 {
  73
  74 /* The thread pool detach()s its threads, so that the threads will not
  75    prevent the process from exiting.  However, it was discovered that
  76    if any detached threads were still waiting on a condition variable,
  77    then the condition variable's destructor would wait for the threads
  78    to exit -- defeating the purpose.
  79
  80    Allocating the thread pool on the heap and simply "leaking" it
  81    avoids this problem.
  82 */
  83 thread_pool *thread_pool::g_thread_pool = new thread_pool ();
  84
  85 thread_pool::~thread_pool ()
  86 {
  87   /* Because this is a singleton, we don't need to clean up.  The
  88      threads are detached so that they won't prevent process exit.
  89      And, cleaning up here would be actively harmful in at least one
  90      case -- see the comment by the definition of g_thread_pool.  */
  91 }
  92
  93 void
  94 thread_pool::set_thread_count (size_t num_threads)
  95 {
  96   std::lock_guard<std::mutex> guard (m_tasks_mutex);
  97
  98   /* If the new size is larger, start some new threads.  */
  99   if (m_thread_count < num_threads)
 100     {
 101       /* Ensure that signals used by gdb are blocked in the new
 102          threads.  */
 103       block_signals blocker;
 104       for (size_t i = m_thread_count; i < num_threads; ++i)
 105         {
 106           try
 107             {
 108               std::thread thread (&thread_pool::thread_function, this);
 109               thread.detach ();
 110             }
 111           catch (const std::system_error &)
 112             {
 113               /* libstdc++ may not implement std::thread, and will
 114                  throw an exception on use.  It seems fine to ignore
 115                  this, and any other sort of startup failure here.  */
 116               num_threads = i;
 117               break;
 118             }
 119         }
 120     }
 121   /* If the new size is smaller, terminate some existing threads.  */
 122   if (num_threads < m_thread_count)
 123     {
 124       for (size_t i = num_threads; i < m_thread_count; ++i)
 125         m_tasks.emplace ();
 126       m_tasks_cv.notify_all ();
 127     }
 128
 129   m_thread_count = num_threads;
 130 }
 131
 132 std::future<void>
 133 thread_pool::post_task (std::function<void ()> func)
 134 {
 135   std::packaged_task<void ()> t (func);
 136   std::future<void> f = t.get_future ();
 137
 138   if (m_thread_count == 0)
 139     {
 140       /* Just execute it now.  */
 141       t ();
 142     }
 143   else
 144     {
 145       std::lock_guard<std::mutex> guard (m_tasks_mutex);
 146       m_tasks.emplace (std::move (t));
 147       m_tasks_cv.notify_one ();
 148     }
 149   return f;
 150 }
 151
 152 void
 153 thread_pool::thread_function ()
 154 {
 155 #ifdef USE_PTHREAD_SETNAME_NP
 156   /* This must be done here, because on macOS one can only set the
 157      name of the current thread.  */
 158   set_thread_name (pthread_setname_np, "gdb worker");
 159 #endif
 160
 161   /* Ensure that SIGSEGV is delivered to an alternate signal
 162      stack.  */
 163   gdb::alternate_signal_stack signal_stack;
 164
 165   while (true)
 166     {
 167       optional<task> t;
 168
 169       {
 170         /* We want to hold the lock while examining the task list, but
 171            not while invoking the task function.  */
 172         std::unique_lock<std::mutex> guard (m_tasks_mutex);
 173         while (m_tasks.empty ())
 174           m_tasks_cv.wait (guard);
 175         t = std::move (m_tasks.front());
 176         m_tasks.pop ();
 177       }
 178
 179       if (!t.has_value ())
 180         break;
 181       (*t) ();
 182     }
 183 }
 184
 185 }
 186
 187 #endif /* CXX_STD_THREAD */