On Fri, 24 May 2024 21:32:08 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > On Tue, 7 May 2024 23:09:22 +0900 > "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote: > > > @@ -109,6 +244,21 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, > > if (!current->ret_stack) > > return -EBUSY; > > > > + /* > > + * At first, check whether the previous fgraph callback is pushed by > > + * the fgraph on the same function entry. > > + * But if @func is the self tail-call function, we also need to ensure > > + * the ret_stack is not for the previous call by checking whether the > > + * bit of @fgraph_idx is set or not. > > + */ > > + ret_stack = get_ret_stack(current, current->curr_ret_stack, &offset); > > + if (ret_stack && ret_stack->func == func && > > + get_fgraph_type(current, offset + FGRAPH_FRAME_OFFSET) == FGRAPH_TYPE_BITMAP && > > + !is_fgraph_index_set(current, offset + FGRAPH_FRAME_OFFSET, fgraph_idx)) > > + return offset + FGRAPH_FRAME_OFFSET; > > + > > + val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET; > > + > > BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long)); > > I'm trying to figure out what the above is trying to do. This gets called > once in function_graph_enter() (or function_graph_enter_ops()). What > exactly are you trying to catch here? Aah, good catch! This was originally for catching the self tail-call case with multiple fgraph callback on the same function, but it was my misread. In later patch ([12/36]), we introduced function_graph_enter_ops() so that we can skip checking hash table and directly pass the fgraph_ops to user callback. I thought this function_graph_enter_ops() is used even if multiple fgraph is set on the same function. In this case, we always need to check the stack can be reused(pushed by other fgraph_ops on the same function) or not. But as we discussed, the function_graph_enter_ops() is used only when only one fgraph is set on the function (if there are multiple fgraphs are set on the same function, use function_graph_enter() ), we are sure that ftrace_push_return_trace() is called only once on hooking the function entry. Thus we don't need to reuse it. > > Is it from this email: > > https://lore.kernel.org/all/20231110105154.df937bf9f200a0c16806c522@xxxxxxxxxx/ > > As that's the last version before you added the above code. > > But you also noticed it may not be needed, but triggered a crash without it > in v3: > > https://lore.kernel.org/all/20231205234511.3839128259dfec153ea7da81@xxxxxxxxxx/ > > I removed this code in my version and it runs just fine. Perhaps there was > another bug that this was hiding that you fixed in later versions? No problem. I think we can remove this block safely. Thank you, > > -- Steve > -- Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
