On 08/30, Roland McGrath wrote: > > > This case is not unlikely, for example ENGINE_STOP is cleared after the > > previous wakeup. We are running, if ENGINE_STOP is set, UTRACE_STOP was > > used and utrace_reset() was called after that. Otherwise, _perhaps_ it > > was cleared by UTRACE_DETACH. > > What we want is to change things so that this case *is* unlikely. That is, > we really want to avoid any extra O(n) work such as utrace_reset calls. It > should never be common to do one of those we don't really want to do. And, > all else being equal, when some extra O(n) work is unavoidably required, > IMHO it's better that it be in the tracer than in the tracee.
Yes, agreed. > I took a different tack and committed these: > > 96fe3cc Revert "utrace_stop: fix UTRACE_DETACH race" > d1a14ce utrace: change UTRACE_STOP bookkeeping > 64a8ca3 utrace_reset cleanup I think this should work, but I'd like to re-read these changes carefully, this all is subtle. And, at first glance, we need something like the patch below. Because now utrace_control(UTRACE_DETACH) does case UTRACE_DETACH: ... if (engine_wants_stop(engine)) resume = true; OK, we need to call utrace_reset() to recalc ->utrace_flags. But utrace_reset() assumes it is safe to play with utrace->attached list, now this is not true. IOW, with this patch utrace_control(UTRACE_DETACH)->utrace_reset() can race with REPORT_CALLBACKS/etc which does list_for_each_entry(->attached). (minor, but perhaps resume should be renamed to "bool reset") "task != current" before utrace_wakeup() doesn't look right too, I think we should check ->stopped instead. Harmless, but otherwise we can call wakeup() while it is not needed. Oleg. --- a/kernel/utrace.c +++ b/kernel/utrace.c @@ -722,11 +722,12 @@ static bool utrace_reset(struct task_str __releases(utrace->lock) { struct utrace_engine *engine, *next; + bool xxx = (utrace->stopped || task == current); unsigned long flags = 0; LIST_HEAD(detached); - splice_attaching(utrace); - + if (xxx) + splice_attaching(utrace); /* * Update the set of events of interest from the union * of the interests of the remaining tracing engines. @@ -734,7 +735,7 @@ static bool utrace_reset(struct task_str * We'll collect them on the detached list. */ list_for_each_entry_safe(engine, next, &utrace->attached, entry) { - if (engine->ops == &utrace_detached_ops) { + if (xxx && engine->ops == &utrace_detached_ops) { engine->ops = NULL; list_move(&engine->entry, &detached); } else { @@ -765,7 +766,7 @@ static bool utrace_reset(struct task_str */ utrace->interrupt = utrace->report = utrace->signal_handler = 0; - if (!(flags & ENGINE_STOP) && task != current) + if (!(flags & ENGINE_STOP) && utrace->stopped) /* * No more engines want it stopped. Wake it up. */