On Thu, Mar 16, 2023 at 12:43 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Tue, 7 Feb 2023 19:21:29 +0100 > Florent Revest <revest@xxxxxxxxxxxx> wrote: > > > @@ -5445,6 +5445,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > /* Enable the tmp_ops to have the same functions as the direct ops */ > > ftrace_ops_init(&tmp_ops); > > tmp_ops.func_hash = ops->func_hash; > > + tmp_ops.direct_call = addr; > > > > err = register_ftrace_function_nolock(&tmp_ops); > > if (err) > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > entry->direct = addr; > > } > > } > > + WRITE_ONCE(ops->direct_call, addr); > > I'm curious about the use of WRITE_ONCE(). It should not go outside the > mutex barrier. This WRITE_ONCE was originally suggested by Mark here: https://lore.kernel.org/all/Y9vW99htjOphDXqY@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/#t My understanding is that it's not so much about avoiding re-ordering but rather about avoiding store tearing since a ftrace_caller trampoline could concurrently read ops->direct_call. Does that make sense ? > -- Steve > > > > > mutex_unlock(&ftrace_lock); > >
