On 09/21, Roland McGrath wrote: > > > OK, so what should we do for now? > > If we can't come to a straightforward answer for the general case > fairly quickly, then we can do the simple change to start with.
I think we should start with changing utrace_control(DETACH) anyway, then try to improve. I'll ressend the one-liner I already showed. > > Without the multitracing utrace_resume()->user_disable_single_step() > > fixes the problem. Otherwise we probably need ENGINE_WANTS_SINGLESTEP. > > No, no, we don't want that. We don't need more state. And, remember, > we really don't need to microoptimize cases when single-step was used > recently--the cost of taking one more single-step trap and percolating > through the signal paths was going to be pretty large anyway. OK, > It's better to have a spurious report (preferably just spurious > TIF_NOTIFY_RESUME with no actual callbacks) following any detach, > or whatever it takes to ensure user_disable_single_step() always > runs if user_enable_*_step did before and there is no engine around > to see a report_quiesce callback pass. If there is a report_quiesce > or report_signal callback pass where nobody returns UTRACE_*STEP, > then we should never leave stepping enabled when we return to user mode. Hmm. I'll try to think more. Right now I don't really understand how to do this correctly. OK, finish_callback_report() and utrace_control(DETACH) can set TIF_NOTIFY_RESUME. But what if there are no more attached engines? Looks like, utrace_resume(UTRACE_RESUME) needs to handle this special case. And utrace_reset() shouldn't clear task->utrace_flags, otherwise utrace_resume/utrace_get_signal won't be called. So, probably detach should set TIF_NOTIFY_RESUME, but utrace_reset() should do user_disable_single_step() too if no more engines. Confused. And in fact I don't understand why this is important. When it comes to multiracing, any engine can hit the unwanted/unexpected trap because another engine can ask for UTRACE_*STEP. The only really important (I think) case is when the last engine detaches. IOW. Suppose that eninge E does utrace_control(STEP) or its callback returns UTRACE_*STEP. If we do not detach this engine, other engines will see the trap. So why it is so important to clear X86_EFLAGS_TF if we detach E ? Oleg.
