GStreamer WebKit debugging by instrumenting source code (2/3)

In this post I show some more useful debugging tricks. Check also the other posts of the series:

Print current thread id

The thread id is generated by Linux and can take values higher than 1-9, just like PIDs. This thread number is useful to know which function calls are issued by the same thread, avoiding confusion between threads.

#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

printf("%s [%d]\n", __PRETTY_FUNCTION__, syscall(SYS_gettid));

Debug GStreamer thread locks

We redefine the GST_OBJECT_LOCK/UNLOCK/TRYLOCK macros to print the calls, compare locks against unlocks, and see who’s not releasing its lock:

#include "wtf/Threading.h"
#define GST_OBJECT_LOCK(obj) do { \
  printf("### [LOCK] %s [%p]\n", __PRETTY_FUNCTION__, &Thread::current()); fflush(stdout); \
  g_mutex_lock(GST_OBJECT_GET_LOCK(obj)); \
} while (0)
#define GST_OBJECT_UNLOCK(obj) do { \
  printf("### [UNLOCK] %s [%p]\n", __PRETTY_FUNCTION__, &Thread::current()); fflush(stdout); \
  g_mutex_unlock(GST_OBJECT_GET_LOCK(obj)); \
} while (0)
#define GST_OBJECT_TRYLOCK(obj) ({ \
  gboolean result = g_mutex_trylock(GST_OBJECT_GET_LOCK(obj)); \
  if (result) { \
   printf("### [LOCK] %s [%p]\n", __PRETTY_FUNCTION__, &Thread::current()); fflush(stdout); \
  } \
  result; \

Warning: The statement expression that allows the TRYLOCK macro to return a value will only work on GCC.

There’s a way to know which thread has taken a lock in glib/GStreamer using gdb. First locate the stalled thread:

(gdb) thread 
(gdb) bt
#2  0x74f07416 in pthread_mutex_lock ()
#3  0x7488aec6 in gst_pad_query ()
#4  0x6debebf2 in autoplug_query_allocation ()

(gdb) frame 3
#3  0x7488aec6 in gst_pad_query (pad=pad@entry=0x54a9b8, ...)
4058        GST_PAD_STREAM_LOCK (pad);

Now get the process id (PID) and use the pthread_mutex_t structure to print the Linux thread id that has acquired the lock:

(gdb) call getpid()
$30 = 6321
(gdb) p ((pthread_mutex_t*)pad.stream_rec_lock.p)->__data.__owner
$31 = 6368
(gdb) thread find 6321.6368
Thread 21 has target id 'Thread 6321.6368'

Trace function calls (poor developer version)

If you’re using C++, you can define a tracer class. This is for webkit, but you get the idea:

#define MYTRACER() MyTracer(__PRETTY_FUNCTION__);
class MyTracer {
    MyTracer(const gchar* functionName)
      : m_functionName(functionName) {
      printf("### %s : begin %d\n", m_functionName.utf8().data(), currentThread()); fflush(stdout);
    virtual ~MyTracer() {
        printf("### %s : end %d\n", m_functionName.utf8().data(), currentThread()); fflush(stdout);
    String m_functionName;

And use it like this in all the functions you want to trace:

void somefunction() {
  // Some other code...

The constructor will log when the execution flow enters into the function and the destructor will log when the flow exits.

Setting breakpoints from C

In the C code, just call raise(SIGINT) (simulate CTRL+C, normally the program would finish).

And then, in a previously attached gdb, after breaking and having debugging all you needed, just continue the execution by ignoring the signal or just plainly continuing:

(gdb) signal 0
(gdb) continue

There’s a way to do the same but attaching gdb after the raise. Use raise(SIGSTOP) instead (simulate CTRL+Z). Then attach gdb, locate the thread calling raise and switch to it:

(gdb) thread apply all bt
[now search for "raise" in the terminal log]
Thread 36 (Thread 1977.2033): #1 0x74f5b3f2 in raise () from /home/enrique/buildroot/output2/staging/lib/
(gdb) thread 36

Now, from a terminal, send a continuation signal: kill -SIGCONT 1977. Finally instruct gdb to single-step only the current thread (IMPORTANT!) and run some steps until all the raises have been processed:

(gdb) set scheduler-locking on
(gdb) next    // Repeat several times...

Know the name of a GStreamer function stored in a pointer at runtime

Just use this macro:


Detecting memory leaks in WebKit

RefCountedLeakCounter is a tool class that can help to debug reference leaks by printing this kind of messages when WebKit exits:

  LEAK: 2 XMLHttpRequest
  LEAK: 25 CachedResource
  LEAK: 3820 WebCoreNode

To use it you have to modify the particular class you want to debug:

  • Include wtf/RefCountedLeakCounter.h
  • DEFINE_DEBUG_ONLY_GLOBAL(WTF::RefCountedLeakCounter, myClassCounter, ("MyClass"));
  • In the constructor: myClassCounter.increment()
  • In the destructor: myClassCounter.decrement()

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