[PATCH hci-8.0 07/12] util/qemu-thread: reserve a signal for thread introspection #VSTOR-144507

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

qemu_thread_create() blocks every signal in the threads it starts so
that process directed signals reach the main loop and nowhere else.
That reasoning covers delivery the kernel is free to route: kill(2)
picks any thread not blocking the signal, so leaving SIGTERM or SIGCHLD
unblocked in a worker would let signalfd miss them.

A thread directed signal is not routed. tgkill(2) delivers to the named
thread whatever every other mask says, so a signal only ever sent that
way was never the main loop's to receive, and unblocking it takes
nothing from it.

Reserve SIGRTMIN for asking a thread what it is doing, and leave that
one unblocked. Nothing sends it yet, and until something installs a
handler the change is invisible: a real time signal terminates by
default, which is already what a stray one does through the main
thread, where it was never blocked either.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Cc: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 include/qemu/osdep.h     | 9 +++++++++
 util/qemu-thread-posix.c | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index e118f12693b..31a13926013 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -644,6 +644,15 @@ int qemu_get_thread_id(void);
  */
 bool qemu_debugger_attached(void);
 
+/*
+ * Signal reserved for asking a thread what it is doing. It is only ever
+ * sent thread directed, so it does not compete for the process directed
+ * signals that qemu routes to the main loop.
+ */
+#if defined(SIGRTMIN) && !defined(_WIN32)
+#define QEMU_SIG_INTROSPECT SIGRTMIN
+#endif
+
 /**
  * qemu_thread_states:
  *
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 013436a51e2..3515d47d788 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -423,6 +423,10 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     sigdelset(&set, SIGFPE);
     sigdelset(&set, SIGILL);
     /* TODO avoid SIGBUS loss on macOS */
+#ifdef QEMU_SIG_INTROSPECT
+    /* Thread directed, so it was never the iothread's to receive */
+    sigdelset(&set, QEMU_SIG_INTROSPECT);
+#endif
     pthread_sigmask(SIG_SETMASK, &set, &oldset);
 
     qemu_thread_args = g_new0(QemuThreadArgs, 1);
-- 
2.53.0