I've been able to reproduce this in my recent kdenlive build using the same MLT/ffmpeg libraries.

I'll be building the latest ffmpeg and MLT from HEAD later. If they appear to be stable and usable I'll re-test against the new MLT with openshot and kdenlive.

ps -em -o pcpu,lwp,stat,pid,tid,class,psr,wchan,args

 103     - -    20387     - -     - -      /usr/bin/kdenlive
 6.7 20387 Sl+      - 20387 TS    0 poll   -
 0.0 20405 Sl+      - 20405 TS    1 futex_ -
 0.0 20406 Sl+      - 20406 TS    1 futex_ -
 0.0 20407 Sl+      - 20407 TS    0 futex_ -
 0.0 20408 Sl+      - 20408 TS    0 futex_ -
 1.2 20412 Sl+      - 20412 TS    0 futex_ -
 0.0 20449 Sl+      - 20449 TS    0 futex_ -
48.8 20450 Sl+      - 20450 TS    0 futex_ -
 0.5 20451 Sl+      - 20451 TS    0 poll   -
61.0 20452 Rl+      - 20452 TS    1 -      -
 1.1 20453 Sl+      - 20453 TS    1 futex_ -

Lightweight thread (LWP) 20452 is running, and pushing the CPU core to 61%. LWP  20450 is sleeping on a futex.

(gdb) ^C
Program received signal SIGINT, Interrupt.
[Switching to Thread 0x7f6001f7f750 (LWP 20387)]
0x00007f5ffdc41496 in *__GI___poll (fds=0x3bb60a0, nfds=5, timeout=299) at ../sysdeps/unix/sysv/linux/poll.c:87
87      ../sysdeps/unix/sysv/linux/poll.c: No such file or directory.
        in ../sysdeps/unix/sysv/linux/poll.c

(gdb) info threads
  64 Thread 0x7f5fd180a950 (LWP 20453)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  63 Thread 0x7f5fd700e950 (LWP 20452)  0x00007f5ff101e390 in ?? () from /usr/lib/libpulse.so.0
  62 Thread 0x7f5fd200b950 (LWP 20451)  0x00007f5ffdc41496 in *__GI___poll (fds=0x621fbd0, nfds=2, timeout=332)
    at ../sysdeps/unix/sysv/linux/poll.c:87
  61 Thread 0x7f5fe03d3950 (LWP 20450)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  60 Thread 0x7f5fe0bd4950 (LWP 20449)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  23 Thread 0x7f5fd680d950 (LWP 20412)  pthread_cond_timedwait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:217
  19 Thread 0x7f5fdebd0950 (LWP 20408)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  18 Thread 0x7f5fde3cf950 (LWP 20407)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  17 Thread 0x7f5fdfbd2950 (LWP 20406)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  16 Thread 0x7f5fdf3d1950 (LWP 20405)  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
* 1 Thread 0x7f6001f7f750 (LWP 20387)  0x00007f5ffdc41496 in *__GI___poll (fds=0x3bb60a0, nfds=5, timeout=299)
    at ../sysdeps/unix/sysv/linux/poll.c:87

(gdb) thread 63
[Switching to thread 63 (Thread 0x7f5fd700e950 (LWP 20452))]#0  0x00007f5ff101e390 in ?? ()
   from /usr/lib/libpulse.so.0
(gdb) bt
#0  0x00007f5ff101e390 in ?? () from /usr/lib/libpulse.so.0
#1  0x00007f5fe18edb48 in ?? () from /usr/lib/alsa-lib/libasound_module_pcm_pulse.so
#2  0x00007f5ff190bdf2 in snd_pcm_ioplug_hw_ptr_update (pcm=0x6224630) at pcm_ioplug.c:56
#3  0x00007f5ff190c685 in snd_pcm_ioplug_avail_update (pcm=0x7c75a60) at pcm_ioplug.c:627
#4  0x00007f5ff18cd8e2 in snd1_pcm_write_areas (pcm=0x6224630, areas=0x7f5fd700dfa0, offset=0, size=512, 
    func=0x7f5ff190c7b0 <ioplug_priv_transfer_areas>) at pcm.c:6627
#5  0x00007f5ff190cb4a in snd_pcm_ioplug_writei (pcm=0x6224630, buffer=<value optimized out>, size=512)
    at pcm_ioplug.c:561
#6  0x00007f5ff1b9454c in ALSA_PlayAudio (this=0x7f5fcc0447e0) at ../../src/audio/alsa/SDL_alsa_audio.c:325
#7  0x00007f5ff1b6708e in SDL_RunAudio (audiop=<value optimized out>) at ../../src/audio/SDL_audio.c:215
#8  0x00007f5ff1b6eb77 in SDL_RunThread (data=0x8282e10) at ../../src/thread/SDL_thread.c:202
#9  0x00007f5ff1bb7e59 in RunThread (data=0x7c75a60) at ../../src/thread/pthread/SDL_systhread.c:47
#10 0x00007f5ffd74c3ba in start_thread (arg=<value optimized out>) at pthread_create.c:297
#11 0x00007f5ffdc4afcd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#12 0x0000000000000000 in ?? ()

(gdb) bt full
#0  0x00007f5ff101e390 in ?? () from /usr/lib/libpulse.so.0
No symbol table info available.
#1  0x00007f5fe18edb48 in ?? () from /usr/lib/alsa-lib/libasound_module_pcm_pulse.so
No symbol table info available.
#2  0x00007f5ff190bdf2 in snd_pcm_ioplug_hw_ptr_update (pcm=0x6224630) at pcm_ioplug.c:56
        io = (ioplug_priv_t *) 0x76d7aa0
        hw = <value optimized out>
#3  0x00007f5ff190c685 in snd_pcm_ioplug_avail_update (pcm=0x7c75a60) at pcm_ioplug.c:627
        io = (ioplug_priv_t *) 0x76d7aa0
        avail = <value optimized out>
#4  0x00007f5ff18cd8e2 in snd1_pcm_write_areas (pcm=0x6224630, areas=0x7f5fd700dfa0, offset=0, size=512, 
    func=0x7f5ff190c7b0 <ioplug_priv_transfer_areas>) at pcm.c:6627
        frames = 512
        avail = 473
        xfer = 0
        err = 0
        state = SND_PCM_STATE_RUNNING
#5  0x00007f5ff190cb4a in snd_pcm_ioplug_writei (pcm=0x6224630, buffer=<value optimized out>, size=512)
    at pcm_ioplug.c:561
No locals.
#6  0x00007f5ff1b9454c in ALSA_PlayAudio (this=0x7f5fcc0447e0) at ../../src/audio/alsa/SDL_alsa_audio.c:325
        status = <value optimized out>
        sample_len = -40516992
        sample_buf = (short int *) 0x76d57b0
#7  0x00007f5ff1b6708e in SDL_RunAudio (audiop=<value optimized out>) at ../../src/audio/SDL_audio.c:215
        audio = (SDL_AudioDevice *) 0x7f5fcc0447e0
        stream = (
    Uint8 *) 0x76d57b0 "\a\nv��\f\210\002P\016�\005\210\016�\a�\r�\a\003\v�\005r\006�\001�\001��\212�����+���R�\006�&�0�M�6�?���F�{�\231�\214���?�\222�G�\230�g���Y���\220��ߴ���w�\177��ܨ���\a�Kܪ�/�\031���"
        stream_len = 2048
        udata = (void *) 0x3a5bb70
---Type <return> to continue, or q <return> to quit---
        fill = (void (*)(void *, Uint8 *, int)) 0x7f5ff1e01eb0 <sdl_fill_audio>
        silence = 0
#8  0x00007f5ff1b6eb77 in SDL_RunThread (data=0x8282e10) at ../../src/thread/SDL_thread.c:202
        userfunc = (int (*)(void *)) 0x7f5ff1b66fd0 <SDL_RunAudio>
        userdata = (void *) 0x7f5fcc0447e0
#9  0x00007f5ff1bb7e59 in RunThread (data=0x7c75a60) at ../../src/thread/pthread/SDL_systhread.c:47
No locals.
#10 0x00007f5ffd74c3ba in start_thread (arg=<value optimized out>) at pthread_create.c:297
        __res = <value optimized out>
        pd = (struct pthread *) 0x7f5fd700e950
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {0, 7241345342359839719, 8392704, 0, 140050326876224, 
        140049769119440, -7295335270796647449, -7295382995374201881}, mask_was_saved = 0}}, priv = {pad = {
      0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = <value optimized out>
        robust = <value optimized out>
#11 0x00007f5ffdc4afcd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
No locals.
#12 0x0000000000000000 in ?? ()
No symbol table info available.

(gdb) thread 61
[Switching to thread 61 (Thread 0x7f5fe03d3950 (LWP 20450))]#0  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
261     ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S: No such file or directory.
        in ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S
Current language:  auto; currently asm
(gdb) bt
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
#1  0x00007f6001b92efd in consumer_read_ahead_thread (arg=<value optimized out>) at mlt_consumer.c:656
#2  0x00007f5ffd74c3ba in start_thread (arg=<value optimized out>) at pthread_create.c:297
#3  0x00007f5ffdc4afcd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#4  0x0000000000000000 in ?? ()