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 <coh...@redhat.com> Suggested-by: Paolo Bonzini <pbonz...@redhat.com> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> --- v3: * Hold trace_lock across fork() to prevent possibility of another thread holding it and disappearing [Paolo] 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