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/* Thread pool
Copyright (C) 2019-2021 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#include "common-defs.h"
#if CXX_STD_THREAD
#include "gdbsupport/thread-pool.h"
#include "gdbsupport/alt-stack.h"
#include "gdbsupport/block-signals.h"
#include <algorithm>
#include <system_error>
/* On the off chance that we have the pthread library on a Windows
host, but std::thread is not using it, avoid calling
pthread_setname_np on Windows. */
#ifndef _WIN32
#ifdef HAVE_PTHREAD_SETNAME_NP
#define USE_PTHREAD_SETNAME_NP
#endif
#endif
#ifdef USE_PTHREAD_SETNAME_NP
#include <pthread.h>
/* Handle platform discrepancies in pthread_setname_np: macOS uses a
single-argument form, while Linux uses a two-argument form. NetBSD
takes a printf-style format and an argument. This wrapper handles the
difference. */
ATTRIBUTE_UNUSED static void
set_thread_name (int (*set_name) (pthread_t, const char *, void *),
const char *name)
{
set_name (pthread_self (), "%s", const_cast<char *> (name));
}
ATTRIBUTE_UNUSED static void
set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
{
set_name (pthread_self (), name);
}
/* The macOS man page says that pthread_setname_np returns "void", but
the headers actually declare it returning "int". */
ATTRIBUTE_UNUSED static void
set_thread_name (int (*set_name) (const char *), const char *name)
{
set_name (name);
}
#endif /* USE_PTHREAD_SETNAME_NP */
namespace gdb
{
/* The thread pool detach()s its threads, so that the threads will not
prevent the process from exiting. However, it was discovered that
if any detached threads were still waiting on a condition variable,
then the condition variable's destructor would wait for the threads
to exit -- defeating the purpose.
Allocating the thread pool on the heap and simply "leaking" it
avoids this problem.
*/
thread_pool *thread_pool::g_thread_pool = new thread_pool ();
thread_pool::~thread_pool ()
{
/* Because this is a singleton, we don't need to clean up. The
threads are detached so that they won't prevent process exit.
And, cleaning up here would be actively harmful in at least one
case -- see the comment by the definition of g_thread_pool. */
}
void
thread_pool::set_thread_count (size_t num_threads)
{
std::lock_guard<std::mutex> guard (m_tasks_mutex);
/* If the new size is larger, start some new threads. */
if (m_thread_count < num_threads)
{
/* Ensure that signals used by gdb are blocked in the new
threads. */
block_signals blocker;
for (size_t i = m_thread_count; i < num_threads; ++i)
{
try
{
std::thread thread (&thread_pool::thread_function, this);
thread.detach ();
}
catch (const std::system_error &)
{
/* libstdc++ may not implement std::thread, and will
throw an exception on use. It seems fine to ignore
this, and any other sort of startup failure here. */
num_threads = i;
break;
}
}
}
/* If the new size is smaller, terminate some existing threads. */
if (num_threads < m_thread_count)
{
for (size_t i = num_threads; i < m_thread_count; ++i)
m_tasks.emplace ();
m_tasks_cv.notify_all ();
}
m_thread_count = num_threads;
}
std::future<void>
thread_pool::post_task (std::function<void ()> func)
{
std::packaged_task<void ()> t (func);
std::future<void> f = t.get_future ();
if (m_thread_count == 0)
{
/* Just execute it now. */
t ();
}
else
{
std::lock_guard<std::mutex> guard (m_tasks_mutex);
m_tasks.emplace (std::move (t));
m_tasks_cv.notify_one ();
}
return f;
}
void
thread_pool::thread_function ()
{
#ifdef USE_PTHREAD_SETNAME_NP
/* This must be done here, because on macOS one can only set the
name of the current thread. */
set_thread_name (pthread_setname_np, "gdb worker");
#endif
/* Ensure that SIGSEGV is delivered to an alternate signal
stack. */
gdb::alternate_signal_stack signal_stack;
while (true)
{
optional<task> t;
{
/* We want to hold the lock while examining the task list, but
not while invoking the task function. */
std::unique_lock<std::mutex> guard (m_tasks_mutex);
while (m_tasks.empty ())
m_tasks_cv.wait (guard);
t = std::move (m_tasks.front());
m_tasks.pop ();
}
if (!t.has_value ())
break;
(*t) ();
}
}
}
#endif /* CXX_STD_THREAD */
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