https://launchpad.net/ubuntu/+source/linux/4.4.0-109.132 :

    linux (4.4.0-109.132) xenial; urgency=low
    
      * linux: 4.4.0-109.132 -proposed tracker (LP: #1742252)
    
      * Kernel trace with xenial 4.4  (4.4.0-108.131, Candidate kernels for PTI fix)
        (LP: #1741934)
        - SAUCE: kaiser: fix perf crashes - fix to original commit

diff'ing the two changes it was this:

    > diff -u linux-4.4.0/arch/x86/events/intel/ds.c linux-4.4.0/arch/x86/events/intel/ds.c
    > --- linux-4.4.0/arch/x86/events/intel/ds.c
    > +++ linux-4.4.0/arch/x86/events/intel/ds.c
    > @@ -415,7 +415,6 @@
    >  		return;
    >  
    >  	per_cpu(cpu_hw_events, cpu).ds = NULL;
    > -	kfree(ds);
    >  }
    >  
    >  void release_ds_buffers(void)

plus it's here: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1741934...