From c25c6c32494c8d1038438b6208d42ba40f25270e Mon Sep 17 00:00:00 2001 From: Pavel Labath Date: Mon, 18 Nov 2024 12:44:06 +0100 Subject: [PATCH] [lldb] Unify/improve MainLoop signal handling (#115197) Change the signal handler to use a pipe to notify about incoming signals. This has two benefits: - the signal no longer has to happen on the MainLoop thread. With the previous implementation, this had to be the case as that was the only way to ensure that ppoll gets interrupted correctly. In a multithreaded process, this would mean that all other threads have to have the signal blocked at all times. - we don't need the android-specific workaround, which was necessary due to the syscall being implemented in a non-atomic way When the MainLoop class was first implemented, we did not have the interrupt self-pipe, so syscall interruption was the most straight-forward implementation. Over time, the class gained new abilities (the pipe being one of them), so we can now piggy-back on those. This patch also changes the kevent-based implementation to use the pipe for signal notification as well. The motivation there is slightly different: - it makes the implementations more uniform - it makes sure we handle all kinds of signals, like we do with the linux version (EVFILT_SIGNAL only catches process-directed signals) --- lldb/include/lldb/Host/posix/MainLoopPosix.h | 1 + lldb/source/Host/posix/MainLoopPosix.cpp | 188 +++++++------------ lldb/unittests/Host/MainLoopTest.cpp | 11 ++ 3 files changed, 81 insertions(+), 119 deletions(-) diff --git a/lldb/include/lldb/Host/posix/MainLoopPosix.h b/lldb/include/lldb/Host/posix/MainLoopPosix.h index 07497b7b8c259a..1988dde7c65aee 100644 --- a/lldb/include/lldb/Host/posix/MainLoopPosix.h +++ b/lldb/include/lldb/Host/posix/MainLoopPosix.h @@ -59,6 +59,7 @@ class MainLoopPosix : public MainLoopBase { private: void ProcessReadObject(IOObject::WaitableHandle handle); void ProcessSignal(int signo); + void ProcessSignals(); class SignalHandle { public: diff --git a/lldb/source/Host/posix/MainLoopPosix.cpp b/lldb/source/Host/posix/MainLoopPosix.cpp index 6f8eaa55cfdf09..040af480771d42 100644 --- a/lldb/source/Host/posix/MainLoopPosix.cpp +++ b/lldb/source/Host/posix/MainLoopPosix.cpp @@ -17,17 +17,14 @@ #include #include #include +#include #include // Multiplexing is implemented using kqueue on systems that support it (BSD -// variants including OSX). On linux we use ppoll, while android uses pselect -// (ppoll is present but not implemented properly). On windows we use WSApoll -// (which does not support signals). +// variants including OSX). On linux we use ppoll. #if HAVE_SYS_EVENT_H #include -#elif defined(__ANDROID__) -#include #else #include #endif @@ -35,11 +32,39 @@ using namespace lldb; using namespace lldb_private; -static sig_atomic_t g_signal_flags[NSIG]; +namespace { +struct GlobalSignalInfo { + sig_atomic_t pipe_fd = -1; + static_assert(sizeof(sig_atomic_t) >= sizeof(int), + "Type too small for a file descriptor"); + sig_atomic_t flag = 0; +}; +} // namespace +static GlobalSignalInfo g_signal_info[NSIG]; static void SignalHandler(int signo, siginfo_t *info, void *) { assert(signo < NSIG); - g_signal_flags[signo] = 1; + + // Set the flag before writing to the pipe! + g_signal_info[signo].flag = 1; + + int fd = g_signal_info[signo].pipe_fd; + if (fd < 0) { + // This can happen with the following (unlikely) sequence of events: + // 1. Thread 1 gets a signal, starts running the signal handler + // 2. Thread 2 unregisters the signal handler, setting pipe_fd to -1 + // 3. Signal handler on thread 1 reads -1 out of pipe_fd + // In this case, we can just ignore the signal because we're no longer + // interested in it. + return; + } + + // Write a(ny) character to the pipe to wake up from the poll syscall. + char c = '.'; + ssize_t bytes_written = llvm::sys::RetryAfterSignal(-1, ::write, fd, &c, 1); + // We can safely ignore EAGAIN (pipe full), as that means poll will definitely + // return. + assert(bytes_written == 1 || (bytes_written == -1 && errno == EAGAIN)); } class MainLoopPosix::RunImpl { @@ -48,7 +73,7 @@ class MainLoopPosix::RunImpl { ~RunImpl() = default; Status Poll(); - void ProcessEvents(); + void ProcessReadEvents(); private: MainLoopPosix &loop; @@ -58,15 +83,9 @@ class MainLoopPosix::RunImpl { struct kevent out_events[4]; int num_events = -1; -#else -#ifdef __ANDROID__ - fd_set read_fd_set; #else std::vector read_fds; #endif - - sigset_t get_sigmask(); -#endif }; #if HAVE_SYS_EVENT_H @@ -94,7 +113,7 @@ Status MainLoopPosix::RunImpl::Poll() { return Status(); } -void MainLoopPosix::RunImpl::ProcessEvents() { +void MainLoopPosix::RunImpl::ProcessReadEvents() { assert(num_events >= 0); for (int i = 0; i < num_events; ++i) { if (loop.m_terminate_request) @@ -103,9 +122,6 @@ void MainLoopPosix::RunImpl::ProcessEvents() { case EVFILT_READ: loop.ProcessReadObject(out_events[i].ident); break; - case EVFILT_SIGNAL: - loop.ProcessSignal(out_events[i].ident); - break; default: llvm_unreachable("Unknown event"); } @@ -113,64 +129,12 @@ void MainLoopPosix::RunImpl::ProcessEvents() { } #else MainLoopPosix::RunImpl::RunImpl(MainLoopPosix &loop) : loop(loop) { -#ifndef __ANDROID__ read_fds.reserve(loop.m_read_fds.size()); -#endif } -sigset_t MainLoopPosix::RunImpl::get_sigmask() { - sigset_t sigmask; - int ret = pthread_sigmask(SIG_SETMASK, nullptr, &sigmask); - assert(ret == 0); - UNUSED_IF_ASSERT_DISABLED(ret); - - for (const auto &sig : loop.m_signals) - sigdelset(&sigmask, sig.first); - return sigmask; -} - -#ifdef __ANDROID__ -Status MainLoopPosix::RunImpl::Poll() { - // ppoll(2) is not supported on older all android versions. Also, older - // versions android (API <= 19) implemented pselect in a non-atomic way, as a - // combination of pthread_sigmask and select. This is not sufficient for us, - // as we rely on the atomicity to correctly implement signal polling, so we - // call the underlying syscall ourselves. - - FD_ZERO(&read_fd_set); - int nfds = 0; - for (const auto &fd : loop.m_read_fds) { - FD_SET(fd.first, &read_fd_set); - nfds = std::max(nfds, fd.first + 1); - } - - union { - sigset_t set; - uint64_t pad; - } kernel_sigset; - memset(&kernel_sigset, 0, sizeof(kernel_sigset)); - kernel_sigset.set = get_sigmask(); - - struct { - void *sigset_ptr; - size_t sigset_len; - } extra_data = {&kernel_sigset, sizeof(kernel_sigset)}; - if (syscall(__NR_pselect6, nfds, &read_fd_set, nullptr, nullptr, nullptr, - &extra_data) == -1) { - if (errno != EINTR) - return Status(errno, eErrorTypePOSIX); - else - FD_ZERO(&read_fd_set); - } - - return Status(); -} -#else Status MainLoopPosix::RunImpl::Poll() { read_fds.clear(); - sigset_t sigmask = get_sigmask(); - for (const auto &fd : loop.m_read_fds) { struct pollfd pfd; pfd.fd = fd.first; @@ -179,55 +143,39 @@ Status MainLoopPosix::RunImpl::Poll() { read_fds.push_back(pfd); } - if (ppoll(read_fds.data(), read_fds.size(), nullptr, &sigmask) == -1 && + if (ppoll(read_fds.data(), read_fds.size(), + /*timeout=*/nullptr, + /*sigmask=*/nullptr) == -1 && errno != EINTR) return Status(errno, eErrorTypePOSIX); return Status(); } -#endif -void MainLoopPosix::RunImpl::ProcessEvents() { -#ifdef __ANDROID__ - // Collect first all readable file descriptors into a separate vector and - // then iterate over it to invoke callbacks. Iterating directly over - // loop.m_read_fds is not possible because the callbacks can modify the - // container which could invalidate the iterator. - std::vector fds; - for (const auto &fd : loop.m_read_fds) - if (FD_ISSET(fd.first, &read_fd_set)) - fds.push_back(fd.first); - - for (const auto &handle : fds) { -#else +void MainLoopPosix::RunImpl::ProcessReadEvents() { for (const auto &fd : read_fds) { if ((fd.revents & (POLLIN | POLLHUP)) == 0) continue; IOObject::WaitableHandle handle = fd.fd; -#endif if (loop.m_terminate_request) return; loop.ProcessReadObject(handle); } - - std::vector signals; - for (const auto &entry : loop.m_signals) - if (g_signal_flags[entry.first] != 0) - signals.push_back(entry.first); - - for (const auto &signal : signals) { - if (loop.m_terminate_request) - return; - g_signal_flags[signal] = 0; - loop.ProcessSignal(signal); - } } #endif MainLoopPosix::MainLoopPosix() : m_triggering(false) { Status error = m_trigger_pipe.CreateNew(/*child_process_inherit=*/false); assert(error.Success()); + + // Make the write end of the pipe non-blocking. + int result = fcntl(m_trigger_pipe.GetWriteFileDescriptor(), F_SETFL, + fcntl(m_trigger_pipe.GetWriteFileDescriptor(), F_GETFL) | + O_NONBLOCK); + assert(result == 0); + UNUSED_IF_ASSERT_DISABLED(result); + const int trigger_pipe_fd = m_trigger_pipe.GetReadFileDescriptor(); m_read_fds.insert({trigger_pipe_fd, [trigger_pipe_fd](MainLoopBase &loop) { char c; @@ -295,7 +243,9 @@ MainLoopPosix::RegisterSignal(int signo, const Callback &callback, sigaddset(&new_action.sa_mask, signo); sigset_t old_set; - g_signal_flags[signo] = 0; + // Set signal info before installing the signal handler! + g_signal_info[signo].pipe_fd = m_trigger_pipe.GetWriteFileDescriptor(); + g_signal_info[signo].flag = 0; // Even if using kqueue, the signal handler will still be invoked, so it's // important to replace it with our "benign" handler. @@ -303,18 +253,7 @@ MainLoopPosix::RegisterSignal(int signo, const Callback &callback, UNUSED_IF_ASSERT_DISABLED(ret); assert(ret == 0 && "sigaction failed"); -#if HAVE_SYS_EVENT_H - struct kevent ev; - EV_SET(&ev, signo, EVFILT_SIGNAL, EV_ADD, 0, 0, 0); - ret = kevent(m_kqueue, &ev, 1, nullptr, 0, nullptr); - assert(ret == 0); -#endif - - // If we're using kqueue, the signal needs to be unblocked in order to - // receive it. If using pselect/ppoll, we need to block it, and later unblock - // it as a part of the system call. - ret = pthread_sigmask(HAVE_SYS_EVENT_H ? SIG_UNBLOCK : SIG_BLOCK, - &new_action.sa_mask, &old_set); + ret = pthread_sigmask(SIG_UNBLOCK, &new_action.sa_mask, &old_set); assert(ret == 0 && "pthread_sigmask failed"); info.was_blocked = sigismember(&old_set, signo); auto insert_ret = m_signals.insert({signo, info}); @@ -349,14 +288,8 @@ void MainLoopPosix::UnregisterSignal( assert(ret == 0); UNUSED_IF_ASSERT_DISABLED(ret); -#if HAVE_SYS_EVENT_H - struct kevent ev; - EV_SET(&ev, signo, EVFILT_SIGNAL, EV_DELETE, 0, 0, 0); - ret = kevent(m_kqueue, &ev, 1, nullptr, 0, nullptr); - assert(ret == 0); -#endif - m_signals.erase(it); + g_signal_info[signo] = {}; } Status MainLoopPosix::Run() { @@ -370,7 +303,9 @@ Status MainLoopPosix::Run() { if (error.Fail()) return error; - impl.ProcessEvents(); + impl.ProcessReadEvents(); + + ProcessSignals(); m_triggering = false; ProcessPendingCallbacks(); @@ -384,6 +319,21 @@ void MainLoopPosix::ProcessReadObject(IOObject::WaitableHandle handle) { it->second(*this); // Do the work } +void MainLoopPosix::ProcessSignals() { + std::vector signals; + for (const auto &entry : m_signals) + if (g_signal_info[entry.first].flag != 0) + signals.push_back(entry.first); + + for (const auto &signal : signals) { + if (m_terminate_request) + return; + + g_signal_info[signal].flag = 0; + ProcessSignal(signal); + } +} + void MainLoopPosix::ProcessSignal(int signo) { auto it = m_signals.find(signo); if (it != m_signals.end()) { diff --git a/lldb/unittests/Host/MainLoopTest.cpp b/lldb/unittests/Host/MainLoopTest.cpp index 4688d4fed475b6..622a547fa22f04 100644 --- a/lldb/unittests/Host/MainLoopTest.cpp +++ b/lldb/unittests/Host/MainLoopTest.cpp @@ -254,6 +254,17 @@ TEST_F(MainLoopTest, Signal) { ASSERT_EQ(1u, callback_count); } +TEST_F(MainLoopTest, SignalOnOtherThread) { + MainLoop loop; + Status error; + + auto handle = loop.RegisterSignal(SIGUSR1, make_callback(), error); + ASSERT_TRUE(error.Success()); + std::thread([] { pthread_kill(pthread_self(), SIGUSR1); }).join(); + ASSERT_TRUE(loop.Run().Success()); + ASSERT_EQ(1u, callback_count); +} + // Test that a signal which is not monitored by the MainLoop does not // cause a premature exit. TEST_F(MainLoopTest, UnmonitoredSignal) {