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: default avatarWei 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.orgSigned-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent c952b35f
......@@ -209,6 +209,18 @@ void machine__exit(struct machine *machine)
for (i = 0; i < THREADS__TABLE_SIZE; i++) {
struct threads *threads = &machine->threads[i];
struct thread *thread, *n;
/*
* Forget about the dead, at this point whatever threads were
* left in the dead lists better have a reference count taken
* by who is using them, and then, when they drop those references
* and it finally hits zero, thread__put() will check and see that
* its not in the dead threads list and will not try to remove it
* from there, just calling thread__delete() straight away.
*/
list_for_each_entry_safe(thread, n, &threads->dead, node)
list_del_init(&thread->node);
exit_rwsem(&threads->lock);
}
}
......@@ -1758,9 +1770,11 @@ static void __machine__remove_thread(struct machine *machine, struct thread *th,
if (threads->last_match == th)
threads__set_last_match(threads, NULL);
BUG_ON(refcount_read(&th->refcnt) == 0);
if (lock)
down_write(&threads->lock);
BUG_ON(refcount_read(&th->refcnt) == 0);
rb_erase_cached(&th->rb_node, &threads->entries);
RB_CLEAR_NODE(&th->rb_node);
--threads->nr;
......@@ -1770,9 +1784,16 @@ static void __machine__remove_thread(struct machine *machine, struct thread *th,
* will be called and we will remove it from the dead_threads list.
*/
list_add_tail(&th->node, &threads->dead);
/*
* We need to do the put here because if this is the last refcount,
* then we will be touching the threads->dead head when removing the
* thread.
*/
thread__put(th);
if (lock)
up_write(&threads->lock);
thread__put(th);
}
void machine__remove_thread(struct machine *machine, struct thread *th)
......
......@@ -125,9 +125,26 @@ void thread__put(struct thread *thread)
{
if (thread && refcount_dec_and_test(&thread->refcnt)) {
/*
* Remove it from the dead_threads list, as last reference
* is gone.
* Remove it from the dead threads list, as last reference is
* gone, if it is in a dead threads list.
*
* We may not be there anymore if say, the machine where it was
* stored was already deleted, so we already removed it from
* the dead threads and some other piece of code still keeps a
* reference.
*
* This is what 'perf sched' does and finally drops it in
* perf_sched__lat(), where it calls perf_sched__read_events(),
* that processes the events by creating a session and deleting
* it, which ends up destroying the list heads for the dead
* threads, but before it does that it removes all threads from
* it using list_del_init().
*
* So we need to check here if it is in a dead threads list and
* if so, remove it before finally deleting the thread, to avoid
* an use after free situation.
*/
if (!list_empty(&thread->node))
list_del_init(&thread->node);
thread__delete(thread);
}
......
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