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