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
set_thread_name(int (* set_name)(pthread_t,const char *,void *),const char * name)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
set_thread_name(int (* set_name)(pthread_t,const char *),const char * name)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
set_thread_name(int (* set_name)(const char *),const char * name)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
~thread_pool()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
set_thread_count(size_t num_threads)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>
post_task(std::function<void ()> func)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
thread_function()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 */
176