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
set_thread_name(int (* set_name)(pthread_t,const char *,void *),const char * name)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
set_thread_name(int (* set_name)(pthread_t,const char *),const char * name)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
set_thread_name(int (* set_name)(const char *),const char * name)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
~thread_pool()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
set_thread_count(size_t num_threads)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>
post_task(std::function<void ()> && func)133 thread_pool::post_task (std::function<void ()> &&func)
134 {
135 std::packaged_task<void ()> t (std::move (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
thread_function()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 */
188