[PATCH 03/10] oslib: add qemu_thread_states()

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

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

A watchdog that reports a hang wants to say what every thread was doing,
and that answer is host specific. Put it beside the other process
introspection helpers rather than in the code that happens to need it
first.

Describe each thread by id, name, scheduler state and the wait channel
it is parked on, all of which Linux exposes in /proc/self/task without
privilege. That is already enough to separate a thread blocked on a
userspace lock, which waits in a futex, from one blocked in the kernel,
which names the primitive it sits on. Skip the caller, which is
reporting rather than hanging.

Hosts with no way to answer return NULL, leaving the caller to print
its own message and nothing more.

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>
Cc: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 include/qemu/osdep.h | 10 +++++++
 util/oslib-posix.c   | 71 ++++++++++++++++++++++++++++++++++++++++++++
 util/oslib-win32.c   |  5 ++++
 3 files changed, 86 insertions(+)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 96afe09632..8039bf91b8 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -667,6 +667,16 @@ int qemu_get_thread_id(void);
  */
 bool qemu_debugger_attached(void);
 
+/**
+ * qemu_thread_states:
+ *
+ * Returns a newly allocated, newline terminated description of every thread
+ * in this process apart from the caller: its id, name, scheduler state and
+ * the wait channel it is parked on, or NULL on a host that cannot report any
+ * of that.
+ */
+char *qemu_thread_states(void);
+
 /**
  * qemu_kill_thread:
  * @tid: thread id.
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 4eb6a40628..79887d370c 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -134,6 +134,77 @@ bool qemu_debugger_attached(void)
 #endif
 }
 
+#ifdef CONFIG_LINUX
+
+static char *read_task_file(const char *tid, const char *name)
+{
+    g_autofree char *path = g_strdup_printf("/proc/self/task/%s/%s", tid, name);
+    char *buf = NULL;
+
+    g_file_get_contents(path, &buf, NULL, NULL);
+    return buf;
+}
+
+static void append_thread_state(GString *out, const char *tid)
+{
+    g_autofree char *comm = read_task_file(tid, "comm");
+    g_autofree char *stat = NULL;
+    g_autofree char *wchan = NULL;
+    char state = '?';
+    char *rparen;
+
+    if (!comm) {
+        return;
+    }
+    g_strchomp(comm);
+
+    stat = read_task_file(tid, "stat");
+    if (stat) {
+        /* comm can hold anything, ')' included, so scan from the right */
+        rparen = strrchr(stat, ')');
+        if (rparen && rparen[1] == ' ') {
+            state = rparen[2];
+        }
+    }
+    g_string_append_printf(out, "  %-7s %-16s %c\n", tid, comm, state);
+
+    wchan = read_task_file(tid, "wchan");
+    if (wchan && wchan[0]) {
+        g_string_append_printf(out, "      wchan %s\n", wchan);
+    }
+}
+
+char *qemu_thread_states(void)
+{
+    g_autoptr(GDir) dir = g_dir_open("/proc/self/task", 0, NULL);
+    const char *tid;
+    GString *out;
+
+    if (!dir) {
+        return NULL;
+    }
+    out = g_string_new(NULL);
+    while ((tid = g_dir_read_name(dir))) {
+        if (atoi(tid) != qemu_get_thread_id()) {
+            append_thread_state(out, tid);
+        }
+    }
+    if (!out->len) {
+        g_string_free(out, TRUE);
+        return NULL;
+    }
+    return g_string_free(out, FALSE);
+}
+
+#else
+
+char *qemu_thread_states(void)
+{
+    return NULL;
+}
+
+#endif /* CONFIG_LINUX */
+
 int qemu_kill_thread(int tid, int sig)
 {
 #if defined(__linux__)
diff --git a/util/oslib-win32.c b/util/oslib-win32.c
index 6663d83151..d18886847c 100644
--- a/util/oslib-win32.c
+++ b/util/oslib-win32.c
@@ -233,6 +233,11 @@ bool qemu_debugger_attached(void)
     return IsDebuggerPresent();
 }
 
+char *qemu_thread_states(void)
+{
+    return NULL;
+}
+
 char *
 qemu_get_local_state_dir(void)
 {
-- 
2.53.0