intel_pt_process_auxtrace_info() creates a pt->unknown_thread thread
that eventually needs to be freed by the last thread__put() on it, when
its refcount hits zero, which may happen in
intel_pt_process_auxtrace_info() error handling path and triggers the
following segfault, which would happen as well at intel_pt_free, when
tools using this intel_pt codebase frees up resources:

  # perf record -I -e intel_pt/tsc=1,noretcomp=1/u /bin/ls
  0  a  anaconda-ks.cfg  bin   perf.data	perf.data.old  perf-f23-bringup.todo
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.217 MB perf.data ]
  #
  # perf script -F event,comm,pid,tid,time,addr,ip,sym,dso,iregs
  Samples for 'instructions:u' event do not have IREGS attribute set. Cannot print 'iregs' field.
  intel_pt_synth_events: failed to synthesize 'instructions' event type
  Segmentation fault (core dumped)
  #

The problem is: there's a union in 'struct thread' combines a list_head
and a rb_node. The standard life cycle of a thread is: init rb_node in
the constructor, insert it into machine->threads rbtree using rb_node,
move it to machine->dead_threads using list_head, clean in the last
thread__put: list_del_init(&thread->node).

In the above command, it clean a thread before adding it into list,
causes the above segfault.

Since pt->unknown_thread will never live in an rbtree, initialize its
list node so that when list_del_init() is done on it we don't segfault.

After this patch:

  # perf script -F event,comm,pid,tid,time,addr,ip,sym,dso,iregs
  Samples for 'instructions:u' event do not have IREGS attribute set. Cannot print 'iregs' field.
  intel_pt_synth_events: failed to synthesize 'instructions' event type
  0x248 [0x88]: failed to process type: 70
  #
Reported-by: Tong Zhang <ztong@vt.edu>
Reported-by: Wang Nan <wangnan0@huawei.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: http://lkml.kernel.org/r/1454296865-19749-1-git-send-email-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>