perf thread: Allow references to thread objects after machine__exit() (4c00af0e) · Commits · 戴 / test

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>

tools/perf/util/machine.c

+23 −2

Original line number	Diff line number	Diff line
		@@ -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)

tools/perf/util/thread.c

+20 −3

Original line number	Diff line number	Diff line
		@@ -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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