Listing 2. Python Code for GDB Mutex Debugging

from collections import defaultdict

# A dictionary of mutex:owner
mutexOwners = {}

# A dictionary of blocking mutex:(threads..)
threadBlockers = defaultdict(list)

# Print the threads
print "Process threads : "
gdb.execute("info threads")

print "Analysing mutexes..."
# Step through processes running under gdb
for process in gdb.inferiors():

    # Step through each thread in the process
    for thread in process.threads():

        # Examine the thread -- is it blocking on a mutex?
        thread.switch()
        frame = gdb.selected_frame()
        if frame.name() == "__lll_lock_wait":

            # a0 is the first argument passed to the function
            a0 = gdb.parse_and_eval("$a0")
            mutex = int(a0)

            # Make a note of which thread blocks on which mutex
            threadBlockers[mutex].append(thread)

            # Make a note of which thread owns this mutex
            if not mutex in mutexOwners:
                s = gdb.execute("x/4d $a0", to_string=True).split()
                s.reverse()
                mutexOwners[mutex] = int(s[1])

    # Print the results of the analysis
    for mutex in mutexOwners:
        print "  Mutex 0x%x :" % mutex
        print "     -> held by thread : %d" % mutexOwners[mutex]
        s = ["%d" % t.ptid[2] for t in threadBlockers[mutex]]
        print "     -> blocks threads : %s" % ' '.join(s)