From: Davidlohr Bueso Date: Mon, 7 Sep 2020 01:33:26 +0000 (-0700) Subject: fgraph: Convert ret_stack tasklist scanning to rcu X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=40d14da383670db21a09e63d52db8dee9b77741e;p=linux.git 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 Signed-off-by: Davidlohr Bueso Signed-off-by: Steven Rostedt (VMware) --- diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c index 1af321dec0f19..5658f13037b3d 100644 --- a/kernel/trace/fgraph.c +++ b/kernel/trace/fgraph.c @@ -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]);