gdb/gdbsupport/thread-pool.c

   1 /* Thread pool
   2
   3    Copyright (C) 2019-2020 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 /* The macOS man page says that pthread_setname_np returns "void", but
  53    the headers actually declare it returning "int".  */
  54 ATTRIBUTE_UNUSED static void
  55 set_thread_name (int (*set_name) (const char *), const char *name)
  56 {
  57   set_name (name);
  58 }
  59
  60 #endif  /* USE_PTHREAD_SETNAME_NP */
  61
  62 namespace gdb
  63 {
  64
  65 /* The thread pool detach()s its threads, so that the threads will not
  66    prevent the process from exiting.  However, it was discovered that
  67    if any detached threads were still waiting on a condition variable,
  68    then the condition variable's destructor would wait for the threads
  69    to exit -- defeating the purpose.
  70
  71    Allocating the thread pool on the heap and simply "leaking" it
  72    avoids this problem.
  73 */
  74 thread_pool *thread_pool::g_thread_pool = new thread_pool ();
  75
  76 thread_pool::~thread_pool ()
  77 {
  78   /* Because this is a singleton, we don't need to clean up.  The
  79      threads are detached so that they won't prevent process exit.
  80      And, cleaning up here would be actively harmful in at least one
  81      case -- see the comment by the definition of g_thread_pool.  */
  82 }
  83
  84 void
  85 thread_pool::set_thread_count (size_t num_threads)
  86 {
  87   std::lock_guard<std::mutex> guard (m_tasks_mutex);
  88
  89   /* If the new size is larger, start some new threads.  */
  90   if (m_thread_count < num_threads)
  91     {
  92       /* Ensure that signals used by gdb are blocked in the new
  93          threads.  */
  94       block_signals blocker;
  95       for (size_t i = m_thread_count; i < num_threads; ++i)
  96         {
  97           std::thread thread (&thread_pool::thread_function, this);
  98           thread.detach ();
  99         }
 100     }
 101   /* If the new size is smaller, terminate some existing threads.  */
 102   if (num_threads < m_thread_count)
 103     {
 104       for (size_t i = num_threads; i < m_thread_count; ++i)
 105         m_tasks.emplace ();
 106       m_tasks_cv.notify_all ();
 107     }
 108
 109   m_thread_count = num_threads;
 110 }
 111
 112 std::future<void>
 113 thread_pool::post_task (std::function<void ()> func)
 114 {
 115   std::packaged_task<void ()> t (func);
 116   std::future<void> f = t.get_future ();
 117
 118   if (m_thread_count == 0)
 119     {
 120       /* Just execute it now.  */
 121       t ();
 122     }
 123   else
 124     {
 125       std::lock_guard<std::mutex> guard (m_tasks_mutex);
 126       m_tasks.emplace (std::move (t));
 127       m_tasks_cv.notify_one ();
 128     }
 129   return f;
 130 }
 131
 132 void
 133 thread_pool::thread_function ()
 134 {
 135 #ifdef USE_PTHREAD_SETNAME_NP
 136   /* This must be done here, because on macOS one can only set the
 137      name of the current thread.  */
 138   set_thread_name (pthread_setname_np, "gdb worker");
 139 #endif
 140
 141   /* Ensure that SIGSEGV is delivered to an alternate signal
 142      stack.  */
 143   gdb::alternate_signal_stack signal_stack;
 144
 145   while (true)
 146     {
 147       optional<task> t;
 148
 149       {
 150         /* We want to hold the lock while examining the task list, but
 151            not while invoking the task function.  */
 152         std::unique_lock<std::mutex> guard (m_tasks_mutex);
 153         while (m_tasks.empty ())
 154           m_tasks_cv.wait (guard);
 155         t = std::move (m_tasks.front());
 156         m_tasks.pop ();
 157       }
 158
 159       if (!t.has_value ())
 160         break;
 161       (*t) ();
 162     }
 163 }
 164
 165 }
 166
 167 #endif /* CXX_STD_THREAD */