gdbsupport/thread-pool.cc

   1 /* Thread pool
   2
   3    Copyright (C) 2019-2022 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 #include "gdbsupport/thread-pool.h"
  22
  23 #if CXX_STD_THREAD
  24
  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 do_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 do_set_thread_name (int (*set_name) (pthread_t, const char *),
  57                     const char *name)
  58 {
  59   set_name (pthread_self (), name);
  60 }
  61
  62 /* The macOS man page says that pthread_setname_np returns "void", but
  63    the headers actually declare it returning "int".  */
  64 ATTRIBUTE_UNUSED static void
  65 do_set_thread_name (int (*set_name) (const char *), const char *name)
  66 {
  67   set_name (name);
  68 }
  69
  70 static void
  71 set_thread_name (const char *name)
  72 {
  73   do_set_thread_name (pthread_setname_np, name);
  74 }
  75
  76 #elif defined (USE_WIN32API)
  77
  78 #include <windows.h>
  79
  80 typedef HRESULT WINAPI (SetThreadDescription_ftype) (HANDLE, PCWSTR);
  81 static SetThreadDescription_ftype *dyn_SetThreadDescription;
  82 static bool initialized;
  83
  84 static void
  85 init_windows ()
  86 {
  87   initialized = true;
  88
  89   HMODULE hm = LoadLibrary (TEXT ("kernel32.dll"));
  90   if (hm)
  91     dyn_SetThreadDescription
  92       = (SetThreadDescription_ftype *) GetProcAddress (hm,
  93                                                        "SetThreadDescription");
  94
  95   /* On some versions of Windows, this function is only available in
  96      KernelBase.dll, not kernel32.dll.  */
  97   if (dyn_SetThreadDescription == nullptr)
  98     {
  99       hm = LoadLibrary (TEXT ("KernelBase.dll"));
 100       if (hm)
 101         dyn_SetThreadDescription
 102           = (SetThreadDescription_ftype *) GetProcAddress (hm,
 103                                                            "SetThreadDescription");
 104     }
 105 }
 106
 107 static void
 108 do_set_thread_name (const wchar_t *name)
 109 {
 110   if (!initialized)
 111     init_windows ();
 112
 113   if (dyn_SetThreadDescription != nullptr)
 114     dyn_SetThreadDescription (GetCurrentThread (), name);
 115 }
 116
 117 #define set_thread_name(NAME) do_set_thread_name (L ## NAME)
 118
 119 #else /* USE_WIN32API */
 120
 121 static void
 122 set_thread_name (const char *name)
 123 {
 124 }
 125
 126 #endif
 127
 128 #endif /* CXX_STD_THREAD */
 129
 130 namespace gdb
 131 {
 132
 133 /* The thread pool detach()s its threads, so that the threads will not
 134    prevent the process from exiting.  However, it was discovered that
 135    if any detached threads were still waiting on a condition variable,
 136    then the condition variable's destructor would wait for the threads
 137    to exit -- defeating the purpose.
 138
 139    Allocating the thread pool on the heap and simply "leaking" it
 140    avoids this problem.
 141 */
 142 thread_pool *thread_pool::g_thread_pool = new thread_pool ();
 143
 144 thread_pool::~thread_pool ()
 145 {
 146   /* Because this is a singleton, we don't need to clean up.  The
 147      threads are detached so that they won't prevent process exit.
 148      And, cleaning up here would be actively harmful in at least one
 149      case -- see the comment by the definition of g_thread_pool.  */
 150 }
 151
 152 void
 153 thread_pool::set_thread_count (size_t num_threads)
 154 {
 155 #if CXX_STD_THREAD
 156   std::lock_guard<std::mutex> guard (m_tasks_mutex);
 157
 158   /* If the new size is larger, start some new threads.  */
 159   if (m_thread_count < num_threads)
 160     {
 161       /* Ensure that signals used by gdb are blocked in the new
 162          threads.  */
 163       block_signals blocker;
 164       for (size_t i = m_thread_count; i < num_threads; ++i)
 165         {
 166           try
 167             {
 168               std::thread thread (&thread_pool::thread_function, this);
 169               thread.detach ();
 170             }
 171           catch (const std::system_error &)
 172             {
 173               /* libstdc++ may not implement std::thread, and will
 174                  throw an exception on use.  It seems fine to ignore
 175                  this, and any other sort of startup failure here.  */
 176               num_threads = i;
 177               break;
 178             }
 179         }
 180     }
 181   /* If the new size is smaller, terminate some existing threads.  */
 182   if (num_threads < m_thread_count)
 183     {
 184       for (size_t i = num_threads; i < m_thread_count; ++i)
 185         m_tasks.emplace ();
 186       m_tasks_cv.notify_all ();
 187     }
 188
 189   m_thread_count = num_threads;
 190 #else
 191   /* No threads available, simply ignore the request.  */
 192 #endif /* CXX_STD_THREAD */
 193 }
 194
 195 void
 196 thread_pool::do_post_task (std::packaged_task<void ()> &&func)
 197 {
 198   std::packaged_task<void ()> t (std::move (func));
 199
 200 #if CXX_STD_THREAD
 201   if (m_thread_count != 0)
 202     {
 203       std::lock_guard<std::mutex> guard (m_tasks_mutex);
 204       m_tasks.emplace (std::move (t));
 205       m_tasks_cv.notify_one ();
 206     }
 207   else
 208 #endif
 209     {
 210       /* Just execute it now.  */
 211       t ();
 212     }
 213 }
 214
 215 #if CXX_STD_THREAD
 216
 217 void
 218 thread_pool::thread_function ()
 219 {
 220   /* This must be done here, because on macOS one can only set the
 221      name of the current thread.  */
 222   set_thread_name ("gdb worker");
 223
 224   /* Ensure that SIGSEGV is delivered to an alternate signal
 225      stack.  */
 226   gdb::alternate_signal_stack signal_stack;
 227
 228   while (true)
 229     {
 230       optional<task_t> t;
 231
 232       {
 233         /* We want to hold the lock while examining the task list, but
 234            not while invoking the task function.  */
 235         std::unique_lock<std::mutex> guard (m_tasks_mutex);
 236         while (m_tasks.empty ())
 237           m_tasks_cv.wait (guard);
 238         t = std::move (m_tasks.front());
 239         m_tasks.pop ();
 240       }
 241
 242       if (!t.has_value ())
 243         break;
 244       (*t) ();
 245     }
 246 }
 247
 248 #endif /* CXX_STD_THREAD */
 249
 250 } /* namespace gdb */