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[Qemu-devel] [PULL for-3.0 1/1] trace/simple: fix hang in child after fo
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PULL for-3.0 1/1] trace/simple: fix hang in child after fork(2) |
Date: |
Tue, 24 Jul 2018 15:25:04 +0100 |
The simple trace backend spawns a write-out thread which is used to
asynchronously flush the in-memory ring buffer to disk.
fork(2) does not clone all threads, only the thread that invoked
fork(2). As a result there is no write-out thread in the child process!
This causes a hang during shutdown when atexit(3) handler installed by
the simple trace backend waits for the non-existent write-out thread.
This patch uses pthread_atfork(3) to terminate the write-out thread
before fork and restart it in both the parent and child after fork.
This solves a hang in qemu-iotests 147 due to qemu-nbd --fork usage.
Reported-by: Cornelia Huck <address@hidden>
Tested-by: Cornelia Huck <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
Message-id: address@hidden
Suggested-by: Paolo Bonzini <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
trace/simple.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 71 insertions(+), 9 deletions(-)
diff --git a/trace/simple.c b/trace/simple.c
index 701dec639c..a4300b6ff1 100644
--- a/trace/simple.c
+++ b/trace/simple.c
@@ -39,9 +39,11 @@
static GMutex trace_lock;
static GCond trace_available_cond;
static GCond trace_empty_cond;
+static GThread *trace_writeout_thread;
static bool trace_available;
static bool trace_writeout_enabled;
+static bool trace_writeout_running;
enum {
TRACE_BUF_LEN = 4096 * 64,
@@ -142,15 +144,34 @@ static void flush_trace_file(bool wait)
g_mutex_unlock(&trace_lock);
}
-static void wait_for_trace_records_available(void)
+/**
+ * Wait to be kicked by flush_trace_file()
+ *
+ * Returns: true if the writeout thread should continue
+ * false if the writeout thread should terminate
+ */
+static bool wait_for_trace_records_available(void)
{
+ bool running;
+
g_mutex_lock(&trace_lock);
- while (!(trace_available && trace_writeout_enabled)) {
+ for (;;) {
+ running = trace_writeout_running;
+ if (!running) {
+ break;
+ }
+
+ if (trace_available && trace_writeout_enabled) {
+ break;
+ }
+
g_cond_signal(&trace_empty_cond);
g_cond_wait(&trace_available_cond, &trace_lock);
}
trace_available = false;
g_mutex_unlock(&trace_lock);
+
+ return running;
}
static gpointer writeout_thread(gpointer opaque)
@@ -165,9 +186,7 @@ static gpointer writeout_thread(gpointer opaque)
size_t unused __attribute__ ((unused));
uint64_t type = TRACE_RECORD_TYPE_EVENT;
- for (;;) {
- wait_for_trace_records_available();
-
+ while (wait_for_trace_records_available()) {
if (g_atomic_int_get(&dropped_events)) {
dropped.rec.event = DROPPED_EVENT_ID,
dropped.rec.timestamp_ns = get_clock();
@@ -398,18 +417,61 @@ static GThread *trace_thread_create(GThreadFunc fn)
return thread;
}
+#ifndef _WIN32
+static void stop_writeout_thread(void)
+{
+ g_mutex_lock(&trace_lock);
+ trace_writeout_running = false;
+ g_cond_signal(&trace_available_cond);
+ g_mutex_unlock(&trace_lock);
+
+ g_thread_join(trace_writeout_thread);
+ trace_writeout_thread = NULL;
+
+ /* Hold trace_lock across fork! Since threads aren't cloned by fork() the
+ * mutex would be held in the child process and cause a deadlock.
+ * Acquiring the mutex here prevents other threads from being in a
+ * trace_lock critical region when fork() occurs.
+ */
+ g_mutex_lock(&trace_lock);
+}
+
+static void restart_writeout_thread(void)
+{
+ trace_writeout_running = true;
+ trace_writeout_thread = trace_thread_create(writeout_thread);
+ if (!trace_writeout_thread) {
+ warn_report("unable to initialize simple trace backend");
+ }
+
+ /* This relies on undefined behavior in the fork() child (it's fine in the
+ * fork() parent). g_mutex_unlock() on a mutex acquired by another thread
+ * is undefined (see glib documentation).
+ */
+ g_mutex_unlock(&trace_lock);
+}
+#endif /* !_WIN32 */
+
bool st_init(void)
{
- GThread *thread;
-
trace_pid = getpid();
+ trace_writeout_running = true;
- thread = trace_thread_create(writeout_thread);
- if (!thread) {
+ trace_writeout_thread = trace_thread_create(writeout_thread);
+ if (!trace_writeout_thread) {
warn_report("unable to initialize simple trace backend");
return false;
}
+#ifndef _WIN32
+ /* Terminate writeout thread across fork and restart it in parent and
+ * child afterwards.
+ */
+ pthread_atfork(stop_writeout_thread,
+ restart_writeout_thread,
+ restart_writeout_thread);
+#endif
+
atexit(st_flush_trace_buffer);
return true;
}
--
2.17.1