[PATCH hci-8.0 06/12] oslib: report kernel stacks in qemu_thread_states() #VSTOR-144507

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

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

A wait channel names the primitive a thread is parked on but not the
path that got it there, and for a thread wedged in a syscall that path
is the answer.

Read /proc/tid/stack when the report is built, and keep the wait
channel for when it cannot be. The file is restricted to CAP_SYS_ADMIN
in the init namespace, so ask once rather than failing per thread, and
say why the report came back thinner. Skip a running thread, which has
no settled stack to unwind, and cap the frames, because the deep end of
a stack is rarely the part that explains the hang.

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 |  4 ++--
 util/oslib-posix.c   | 52 +++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 51 insertions(+), 5 deletions(-)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index d3e85c606f0..e118f12693b 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -649,8 +649,8 @@ bool qemu_debugger_attached(void);
  *
  * 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.
+ * kernel stack, or NULL on a host that cannot report any of that. Where the
+ * kernel stack is privileged the description falls back to a wait channel.
  */
 char *qemu_thread_states(void);
 
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index c4856f8cba5..d5aa7d398f1 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -145,13 +145,39 @@ static char *read_task_file(const char *tid, const char *name)
     return buf;
 }
 
+#define THREAD_STACK_FRAMES 12
+
+/*
+ * /proc/tid/stack is restricted to CAP_SYS_ADMIN in the init namespace, so
+ * ask once instead of failing per thread, and say why the report is thinner.
+ */
+static bool thread_stacks_readable(void)
+{
+    static int readable = -1;
+    g_autofree char *path = NULL;
+    g_autofree char *buf = NULL;
+
+    if (readable >= 0) {
+        return readable;
+    }
+    path = g_strdup_printf("/proc/self/task/%d/stack", qemu_get_thread_id());
+    readable = g_file_get_contents(path, &buf, NULL, NULL);
+    if (!readable) {
+        warn_report("kernel stacks need CAP_SYS_ADMIN, falling back to wchan");
+    }
+    return readable;
+}
+
 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 *stack = NULL;
     g_autofree char *wchan = NULL;
+    g_auto(GStrv) lines = NULL;
     char state = '?';
     char *rparen;
+    int i;
 
     if (!comm) {
         return;
@@ -168,9 +194,26 @@ static void append_thread_state(GString *out, const char *tid)
     }
     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);
+    /* A running thread has no settled kernel stack to unwind */
+    if (state != 'R' && thread_stacks_readable()) {
+        stack = read_task_file(tid, "stack");
+    }
+    if (!stack) {
+        wchan = read_task_file(tid, "wchan");
+        if (wchan && wchan[0]) {
+            g_string_append_printf(out, "      wchan %s\n", wchan);
+        }
+        return;
+    }
+
+    lines = g_strsplit(stack, "\n", 0);
+    for (i = 0; i < THREAD_STACK_FRAMES && lines[i]; i++) {
+        if (lines[i][0]) {
+            g_string_append_printf(out, "      %s\n", lines[i]);
+        }
+    }
+    if (lines[i]) {
+        g_string_append(out, "      ...\n");
     }
 }
 
@@ -183,6 +226,9 @@ char *qemu_thread_states(void)
     if (!dir) {
         return NULL;
     }
+    /* Emit the permission notice ahead of the listing, not inside it */
+    thread_stacks_readable();
+
     out = g_string_new(NULL);
     while ((tid = g_dir_read_name(dir))) {
         if (atoi(tid) != qemu_get_thread_id()) {
-- 
2.53.0