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 #include "diagnostics.h"
  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 DIAGNOSTIC_PUSH
  44 DIAGNOSTIC_IGNORE_UNUSED_FUNCTION
  45
  46 /* Handle platform discrepancies in pthread_setname_np: macOS uses a
  47    single-argument form, while Linux uses a two-argument form.  This
  48    wrapper handles the difference.  */
  49
  50 static void
  51 set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
  52 {
  53   set_name (pthread_self (), name);
  54 }
  55
  56 static void
  57 set_thread_name (void (*set_name) (const char *), const char *name)
  58 {
  59   set_name (name);
  60 }
  61
  62 DIAGNOSTIC_POP
  63
  64 #endif  /* USE_PTHREAD_SETNAME_NP */
  65
  66 namespace gdb
  67 {
  68
  69 /* The thread pool detach()s its threads, so that the threads will not
  70    prevent the process from exiting.  However, it was discovered that
  71    if any detached threads were still waiting on a condition variable,
  72    then the condition variable's destructor would wait for the threads
  73    to exit -- defeating the purpose.
  74
  75    Allocating the thread pool on the heap and simply "leaking" it
  76    avoids this problem.
  77 */
  78 thread_pool *thread_pool::g_thread_pool = new thread_pool ();
  79
  80 thread_pool::~thread_pool ()
  81 {
  82   /* Because this is a singleton, we don't need to clean up.  The
  83      threads are detached so that they won't prevent process exit.
  84      And, cleaning up here would be actively harmful in at least one
  85      case -- see the comment by the definition of g_thread_pool.  */
  86 }
  87
  88 void
  89 thread_pool::set_thread_count (size_t num_threads)
  90 {
  91   std::lock_guard<std::mutex> guard (m_tasks_mutex);
  92
  93   /* If the new size is larger, start some new threads.  */
  94   if (m_thread_count < num_threads)
  95     {
  96       /* Ensure that signals used by gdb are blocked in the new
  97          threads.  */
  98       block_signals blocker;
  99       for (size_t i = m_thread_count; i < num_threads; ++i)
 100         {
 101           std::thread thread (&thread_pool::thread_function, this);
 102           thread.detach ();
 103         }
 104     }
 105   /* If the new size is smaller, terminate some existing threads.  */
 106   if (num_threads < m_thread_count)
 107     {
 108       for (size_t i = num_threads; i < m_thread_count; ++i)
 109         m_tasks.emplace ();
 110       m_tasks_cv.notify_all ();
 111     }
 112
 113   m_thread_count = num_threads;
 114 }
 115
 116 std::future<void>
 117 thread_pool::post_task (std::function<void ()> func)
 118 {
 119   std::packaged_task<void ()> t (func);
 120   std::future<void> f = t.get_future ();
 121
 122   if (m_thread_count == 0)
 123     {
 124       /* Just execute it now.  */
 125       t ();
 126     }
 127   else
 128     {
 129       std::lock_guard<std::mutex> guard (m_tasks_mutex);
 130       m_tasks.emplace (std::move (t));
 131       m_tasks_cv.notify_one ();
 132     }
 133   return f;
 134 }
 135
 136 void
 137 thread_pool::thread_function ()
 138 {
 139 #ifdef USE_PTHREAD_SETNAME_NP
 140   /* This must be done here, because on macOS one can only set the
 141      name of the current thread.  */
 142   set_thread_name (pthread_setname_np, "gdb worker");
 143 #endif
 144
 145   /* Ensure that SIGSEGV is delivered to an alternate signal
 146      stack.  */
 147   gdb::alternate_signal_stack signal_stack;
 148
 149   while (true)
 150     {
 151       optional<task> t;
 152
 153       {
 154         /* We want to hold the lock while examining the task list, but
 155            not while invoking the task function.  */
 156         std::unique_lock<std::mutex> guard (m_tasks_mutex);
 157         while (m_tasks.empty ())
 158           m_tasks_cv.wait (guard);
 159         t = std::move (m_tasks.front());
 160         m_tasks.pop ();
 161       }
 162
 163       if (!t.has_value ())
 164         break;
 165       (*t) ();
 166     }
 167 }
 168
 169 }
 170
 171 #endif /* CXX_STD_THREAD */