8

最近valgrind --tool=helgrind、自分のプロジェクトを実行したところ、「データ競合の可能性がある」という警告が表示されました。これは懸念事項だと思いました。ただし、この単純なテスト プログラムでも、次のメッセージが表示されます。

#include <iostream>
#include <thread>
#include <future>

int main()
{
   std::packaged_task<void()> task([]()
   {
      std::cout << "Hello\n"; // You can leave this out
   });
   auto future = task.get_future();
   std::thread(std::move(task)).detach();
   future.get();
}

でコンパイルするとg++-4.9 -p -g -std=c++11 -pthread -O3 test.cpp、 の出力valgrind --tool=helgrind ./a.outは次のようになります。

==12808== Helgrind, a thread error detector
==12808== Copyright (C) 2007-2013, and GNU GPL'd, by OpenWorks LLP et al.
==12808== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==12808== Command: ./a.out
==12808== 
Hello
==12808== ---Thread-Announcement------------------------------------------
==12808== 
==12808== Thread #2 was created
==12808==    at 0x567B73E: clone (clone.S:74)
==12808==    by 0x536A1F9: do_clone.constprop.4 (createthread.c:75)
==12808==    by 0x536B7EB: create_thread (createthread.c:245)
==12808==    by 0x536B7EB: pthread_create@@GLIBC_2.2.5 (pthread_create.c:606)
==12808==    by 0x4C30E3D: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4EF7F08: std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x403C3D: std::thread::thread<std::packaged_task<void ()>>(std::packaged_task<void ()>&&) (thread:138)
==12808==    by 0x401D57: main (test.cpp:13)
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Thread #2: pthread_cond_{signal,broadcast}: dubious: associated lock is not held by any thread
==12808==    at 0x4C2F295: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4EF3CE8: std::condition_variable::notify_all() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x402F18: _M_set_result (future:374)
==12808==    by 0x402F18: _M_run (future:1319)
==12808==    by 0x402F18: operator() (future:1453)
==12808==    by 0x402F18: _M_invoke<> (functional:1700)
==12808==    by 0x402F18: operator() (functional:1688)
==12808==    by 0x402F18: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::_M_run() (thread:115)
==12808==    by 0x4EF7DCF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x4C30FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x536B0A4: start_thread (pthread_create.c:309)
==12808==    by 0x567B77C: clone (clone.S:111)
==12808== 
==12808== ---Thread-Announcement------------------------------------------
==12808== 
==12808== Thread #1 is the program's root thread
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Possible data race during read of size 8 at 0x5C4FAA0 by thread #1
==12808== Locks held: none
==12808==    at 0x4026A9: ~unique_ptr (unique_ptr.h:235)
==12808==    by 0x4026A9: ~_Task_state_base (future:1281)
==12808==    by 0x4026A9: ~_Task_state (future:1302)
==12808==    by 0x4026A9: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (new_allocator.h:124)
==12808==    by 0x4026A9: _S_destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:282)
==12808==    by 0x4026A9: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:411)
==12808==    by 0x4026A9: std::_Sp_counted_ptr_inplace<std::__future_base::_Task_state<main::{lambda()#1}, std::allocator<int>, void ()>, main::{lambda()#1}, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (shared_ptr_base.h:524)
==12808==    by 0x401ED4: _M_release (shared_ptr_base.h:149)
==12808==    by 0x401ED4: ~__shared_count (shared_ptr_base.h:666)
==12808==    by 0x401ED4: ~__shared_ptr (shared_ptr_base.h:914)
==12808==    by 0x401ED4: reset (shared_ptr_base.h:1015)
==12808==    by 0x401ED4: ~_Reset (future:657)
==12808==    by 0x401ED4: get (future:786)
==12808==    by 0x401ED4: main (test.cpp:14)
==12808== 
==12808== This conflicts with a previous write of size 8 by thread #2
==12808== Locks held: none
==12808==    at 0x403407: release (unique_ptr.h:328)
==12808==    by 0x403407: unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter, void> (unique_ptr.h:221)
==12808==    by 0x403407: ~packaged_task (future:1409)
==12808==    by 0x403407: ~_Head_base (tuple:128)
==12808==    by 0x403407: ~_Tuple_impl (tuple:229)
==12808==    by 0x403407: ~tuple (tuple:388)
==12808==    by 0x403407: ~_Bind_simple (functional:1663)
==12808==    by 0x403407: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::~_Impl() (thread:107)
==12808==    by 0x4EF7E1A: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x4C30FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x536B0A4: start_thread (pthread_create.c:309)
==12808==    by 0x567B77C: clone (clone.S:111)
==12808==  Address 0x5c4faa0 is 128 bytes inside a block of size 144 alloc'd
==12808==    at 0x4C2C520: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x401C8C: allocate (new_allocator.h:104)
==12808==    by 0x401C8C: allocate (alloc_traits.h:357)
==12808==    by 0x401C8C: __shared_count<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:616)
==12808==    by 0x401C8C: __shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:1090)
==12808==    by 0x401C8C: shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:316)
==12808==    by 0x401C8C: allocate_shared<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:588)
==12808==    by 0x401C8C: __create_task_state<void(), main()::<lambda()>, std::allocator<int> > (future:1351)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, std::allocator<int>, void> (future:1403)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, void> (future:1393)
==12808==    by 0x401C8C: main (test.cpp:11)
==12808==  Block was alloc'd by thread #1
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Possible data race during read of size 1 at 0x5C4FA6C by thread #1
==12808== Locks held: none
==12808==    at 0x4C2ED90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4C2F57F: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4026D7: ~_State_baseV2 (future:316)
==12808==    by 0x4026D7: ~_Task_state_base (future:1281)
==12808==    by 0x4026D7: ~_Task_state (future:1302)
==12808==    by 0x4026D7: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (new_allocator.h:124)
==12808==    by 0x4026D7: _S_destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:282)
==12808==    by 0x4026D7: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:411)
==12808==    by 0x4026D7: std::_Sp_counted_ptr_inplace<std::__future_base::_Task_state<main::{lambda()#1}, std::allocator<int>, void ()>, main::{lambda()#1}, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (shared_ptr_base.h:524)
==12808==    by 0x401ED4: _M_release (shared_ptr_base.h:149)
==12808==    by 0x401ED4: ~__shared_count (shared_ptr_base.h:666)
==12808==    by 0x401ED4: ~__shared_ptr (shared_ptr_base.h:914)
==12808==    by 0x401ED4: reset (shared_ptr_base.h:1015)
==12808==    by 0x401ED4: ~_Reset (future:657)
==12808==    by 0x401ED4: get (future:786)
==12808==    by 0x401ED4: main (test.cpp:14)
==12808== 
==12808== This conflicts with a previous write of size 4 by thread #2
==12808== Locks held: none
==12808==    at 0x536F9BF: pthread_cond_broadcast@@GLIBC_2.3.2 (pthread_cond_broadcast.S:59)
==12808==    by 0x402F18: _M_set_result (future:374)
==12808==    by 0x402F18: _M_run (future:1319)
==12808==    by 0x402F18: operator() (future:1453)
==12808==    by 0x402F18: _M_invoke<> (functional:1700)
==12808==    by 0x402F18: operator() (functional:1688)
==12808==    by 0x402F18: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::_M_run() (thread:115)
==12808==  Address 0x5c4fa6c is 76 bytes inside a block of size 144 alloc'd
==12808==    at 0x4C2C520: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x401C8C: allocate (new_allocator.h:104)
==12808==    by 0x401C8C: allocate (alloc_traits.h:357)
==12808==    by 0x401C8C: __shared_count<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:616)
==12808==    by 0x401C8C: __shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:1090)
==12808==    by 0x401C8C: shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:316)
==12808==    by 0x401C8C: allocate_shared<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:588)
==12808==    by 0x401C8C: __create_task_state<void(), main()::<lambda()>, std::allocator<int> > (future:1351)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, std::allocator<int>, void> (future:1403)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, void> (future:1393)
==12808==    by 0x401C8C: main (test.cpp:11)
==12808==  Block was alloc'd by thread #1
==12808== 
==12808== 
==12808== For counts of detected and suppressed errors, rerun with: -v
==12808== Use --history-level=approx or =none to gain increased speed, at
==12808== the cost of reduced accuracy of conflicting-access information
==12808== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 23 from 23)

私の質問: これを無視しても安全ですか? もしそうなら、これらの警告を抑制する方法は?

4

1 に答える 1

1

私もこの警告に/usr/include/c++/4.8/future出くわし、Kubuntu 14.04 で GCC 4.8 ( ) の libstdc++ のソース コードを確認しました。バグは見つかりませんでした。のfuture状態には、とりわけ、結果ポインタ、ミューテックス、および条件変数が含まれます。future<T>::wait()ミューテックスをロックし、結果ポインタが null かどうかをチェックします。そうであれば、条件変数を待機します (これにより、1 つのアトミック操作でミューテックスもロック解除されます)。でpromise<T>::set_value()(またはset_exception()、私が推測する)、コードはミューテックスをロックし、結果ポインターを設定し、ミューテックスをロック解除notify_all()し、condition variable. が呼び出されたときにミューテックスがロックされていないため、Helgrind は警告しnotify_all()ます。ただし、チェックインの結果を変更する唯一の方法であるポインターを設定するためにミューテックスが保持されているため、コードは依然として安全です。future<T>::wait(). の有効期間外に通知を行うことはlock_guard、最適化になる可能性があります (できるだけ短時間ミューテックスを保持します)。残念ながら、この警告につながります。libstdc++ の保守担当者は、おそらくパッチの受け入れに消極的です。なぜなら、a) パッチはパフォーマンスを少しでも低下させ、b) 壊れていなければ修正しないからです。したがって、ヘルグリンドを制圧することをお勧めします。

明確さと簡潔さのためにいくつかの相互作用を単純化したことに注意してください。特にfuture、promise実際の作業を非表示の実装クラスに委任set_valueし、追加を使用してonce_flag同時呼び出しを同期するなどです。

このスレッド、それに対する私の回答、およびCubbiによる私の回答に対する正確なコメント (私が見落としていたもの) も参照してください。

于 2015-06-03T15:31:17.533 に答える