Commit 40d14da3 authored Sep 06, 2020 by Davidlohr Bueso Committed by Steven Rostedt (VMware) Sep 21, 2020

fgraph: Convert ret_stack tasklist scanning to rcu

It seems that alloc_retstack_tasklist() can also take a lockless
approach for scanning the tasklist, instead of using the big global
tasklist_lock. For this we also kill another deprecated and rcu-unsafe
tsk->thread_group user replacing it with for_each_process_thread(),
maintaining semantics.

Here tasklist_lock does not protect anything other than the list
against concurrent fork/exit. And considering that the whole thing
is capped by FTRACE_RETSTACK_ALLOC_SIZE (32), it should not be a
problem to have a pontentially stale, yet stable, list. The task cannot
go away either, so we don't risk racing with ftrace_graph_exit_task()
which clears the retstack.

The tsk->ret_stack management is not protected by tasklist_lock, being
serialized with the corresponding publish/subscribe barriers against
concurrent ftrace_push_return_trace(). In addition this plays nicer
with cachelines by avoiding two atomic ops in the uncontended case.

Link: https://lkml.kernel.org/r/20200907013326.9870-1-dave@stgolabs.net



Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

parent eb8d8b4c

kernel/trace/fgraph.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -387,8 +387,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
		}
		}

		read_lock(&tasklist_lock);
		do_each_thread(g, t) {
		rcu_read_lock();
		for_each_process_thread(g, t) {
		if (start == end) {
		ret = -EAGAIN;
		goto unlock;
		@@ -403,10 +403,10 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
		smp_wmb();
		t->ret_stack = ret_stack_list[start++];
		}
		} while_each_thread(g, t);
		}

		unlock:
		read_unlock(&tasklist_lock);
		rcu_read_unlock();
		free:
		for (i = start; i < end; i++)
		kfree(ret_stack_list[i]);

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