* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 6 Jan 2009, Linus Torvalds wrote: > > > > So it should be renamed. Something like "task_is_oncpu()" or whatever. > > Another complaint, which is tangentially related in that it actually > concerns "current". > > Right now, if some process deadlocks on a mutex, we get hung process, > but with a nice backtrace and hopefully other things (that don't need > that lock) still continue to work. > > But if I read it correctly, the adaptive spin code will instead just > hang. Exactly because "task_is_current()" will also trigger for that > case, and now you get an infinite loop, with the process spinning until > it looses its own CPU, which obviously will never happen. > > Yes, this is the behavior we get with spinlocks too, and yes, lock > debugging will talk about it, but it's a regression. We've historically > had a _lot_ more bad deadlocks on mutexes than we have had on spinlocks, > exactly because mutexes can be held over much more complex code. So > regressing on it and making it less debuggable is bad. > > IOW, if we do this, then I think we need a > > BUG_ON(task == owner); > > in the waiting slow-path. I realize the test already exists for the > DEBUG case, but I think we just want it even for production kernels. > Especially since we'd only ever need it in the slow-path. yeah, sounds good. One thought: BUG_ON()'s do_exit() shows a slightly misleading failure pattern to users: instead of a 'hanging' task, we'd get a misbehaving app due to one of its tasks exiting spuriously. It can even go completely unnoticed [users dont look at kernel logs normally] - while a hanging task generally does get noticed. (because there's no progress in processing) So instead of the BUG_ON() we could emit a WARN_ONCE() perhaps, plus not do any spinning and just block - resulting in an uninterruptible task (that the user will probably notice) and a scary message in the syslog? [all in the slowpath] So in this case WARN_ONCE() is both more passive (it does not run do_exit()), and shows the more intuitive failure pattern to users. No strong feelings though. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
