Fix: run-as thread deadlock on itself in restart error path
authorJonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Wed, 16 Jan 2019 18:38:57 +0000 (13:38 -0500)
committerJérémie Galarneau <jeremie.galarneau@efficios.com>
Mon, 11 Feb 2019 20:18:59 +0000 (15:18 -0500)
commita01c682b9919f98d45a9127b6e02fd08dcc900bc
tree5d4cd37055c7985dcf6620ce99df27f76449fd30
parent5b8139c62c3bc4a769e3430edb8d4105d2152356
Fix: run-as thread deadlock on itself in restart error path

The deadlock was found using this backtrace

Thread 5:
0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
1  0x00007efc6b650023 in __GI___pthread_mutex_lock (mutex=mutex@entry=0x55fc37128400 <worker_lock>) at ../nptl/pthread_mutex_lock.c:78
2  0x000055fc36efbe05 in run_as_destroy_worker () at runas.c:1233
3  0x000055fc36efc2e7 in run_as_restart_worker (worker=<optimized out>) at runas.c:998
4  run_as (cmd=cmd@entry=RUN_AS_UNLINK, data=data@entry=0x7efc5b7fa630, ret_value=ret_value@entry=0x7efc5b7fa510, uid=uid@entry=1000, gid=gid@entry=1000) at runas.c:1033
5  0x000055fc36efc9ce in run_as_unlink (path=path@entry=0x7efc5b7fb690 "/home/joraj/lttng-traces/auto-20190116-111518/20190116T111729-0500-33/kernel/index/channel0_3.idx", uid=uid@entry=1000, gid=gid@entry=1000) at runas.c :1120
6  0x000055fc36ef7feb in utils_unlink_stream_file (path_name=path_name@entry=0x7efc5b7fc7e0 "/home/joraj/lttng-traces/auto-20190116-111518/20190116T111729-0500-33/kernel/index", file_name=file_name@entry=0x7efc500085d4 "channel0_3", size=size@entry=0, count=count@entry=0, uid=uid@entry=1000, gid=gid@entry=1000, suffix=0x55fc36f19b26 ".idx") at utils.c:929
7  0x000055fc36f01d4e in lttng_index_file_create (path_name=path_name@entry=0x7efc500087a0 "/home/joraj/lttng-traces/auto-20190116-111518/20190116T111729-0500-33/kernel", stream_name=stream_name@entry=0x7efc500085d4 "channel0_3", uid=1000, gid=1000, size=0, count=0, major=1, minor=1) at index.c:79
8  0x000055fc36ed9475 in rotate_local_stream (ctx=<optimized out>, stream=0x7efc50008460) at consumer.c:4105
9  0x000055fc36ed98b5 in lttng_consumer_rotate_stream (ctx=ctx@entry=0x55fc37428d80, stream=stream@entry=0x7efc50008460, rotated=rotated@entry=0x7efc5b7fdb27) at consumer.c:4181
10 0x000055fc36ee354e in lttng_kconsumer_read_subbuffer (stream=stream@entry=0x7efc50008460, ctx=ctx@entry=0x55fc37428d80, rotated=rotated@entry=0x7efc5b7fdb27) at kernel-consumer.c:1740
11 0x000055fc36ed7a30 in lttng_consumer_read_subbuffer (stream=0x7efc50008460, ctx=0x55fc37428d80) at consumer.c:3383
12 0x000055fc36ed4b74 in consumer_thread_data_poll (data=0x55fc37428d80) at consumer.c:2751
13 0x00007efc6b64d6db in start_thread (arg=0x7efc5b7fe700) at pthread_create.c:463
14 0x00007efc6af6488f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

The owner of the lock is itself:
  print worker_lock.__data.__owner
    $2 = 25725
  thread find 25725
    Thread 5 has target id 'Thread 0x7efc5b7fe700 (LWP 25725)'

The worker_lock is first taken in frame #4: run_as runas.c:1033

  pthread_mutex_lock(&worker_lock);
  if (use_clone()) {
...
    /*
     * If the worker thread crashed the errno is set to EIO. we log
     * the error and  start a new worker process.
     */
    if (ret == -1 && saved_errno == EIO) {
        DBG("Socket closed unexpectedly... "
        "Restarting the worker process");
->      ret = run_as_restart_worker(global_worker);
        if (ret == -1) {
          ERR("Failed to restart worker process.");
          goto err;
        }

Solution
========

Create run_as_restart_worker_no_lock which does not to take the lock on
execution.
Use run_as_restart_worker_no_lock at the run_as error path call site.
Use run_as_restart_worker_no_lock inside run_as_restart_worker while
holding the worker lock to provide identical behaviour to other call sites.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
src/common/runas.c
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