perf thread: Allow references to thread objects after machine__exit()

perf thread: Allow references to thread objects after machine__exit()

Threads are created when we either synthesize PERF_RECORD_FORK events
for pre-existing threads or when we receive PERF_RECORD_FORK events from
the kernel as new threads get created.

We then keep them in machine->threads[].entries rb trees till when we
receive a PERF_RECORD_EXIT, i.e. that thread terminated.

The thread object has a reference count that is grabbed when, for
instance, we keep that thread referenced in struct hist_entry, in 'perf
report' and 'perf top'.

When we receive a PERF_RECORD_EXIT we remove the thread object from the
rb tree and move it to the corresponding machine->threads[].dead list,
then we do a thread__put(), dropping the reference we had for keeping it
in the rb tree.

In thread__put() we were assuming that when the reference count hit zero
we should remove it from the dead list by simply doing a
list_del_init(&thread->node).

That works well when all the thread lifetime is during the machine that
has the list heads lifetime, since we know that we can do the
list_del_init() and it will update the 'dead' list_head.

But in 'perf sched lat' we were doing:

    machine__new() (via perf_session__new)

    process events, grabbing refcounts to keep those thread objects
    in 'perf sched' local data structures.

    machine__exit() (via perf_session__delete) which would delete the
    'dead' list heads.

    And then doing the final thread__put() for the refcounts 'perf sched'
    rightfully obtained for keeping those thread object references.

    b00m, since thread__put() would do the list_del_init() touching
    a dead dead list head.

Fix it by removing all the dead threads from machine->threads[].dead at
machine__exit(), since whatever is there should have refcounts taken by
things like 'perf sched lat', and make thread__put() check if the thread
is in a linked list before removing it from that list.

Reported-by: Wei Li <liwei391@huawei.com>
Link: https://lkml.kernel.org/r/20190508143648.8153-1-liwei391@huawei.com
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zhipeng Xie <xiezhipeng1@huawei.com>
Link: https://lkml.kernel.org/r/20190704194355.GI10740@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>