On Mon, Jun 25, 2018 at 05:28:24PM +0900, Byungchul Park wrote: > On Fri, Jun 22, 2018 at 01:05:48PM -0700, Joel Fernandes wrote: > > On Fri, Jun 22, 2018 at 02:32:47PM -0400, Steven Rostedt wrote: > > > On Fri, 22 Jun 2018 11:19:16 -0700 > > > Joel Fernandes <j...@joelfernandes.org> wrote: > > > > > > > Sure. So in a later thread you mentioned "usermode helpers". I took a > > > > closer > > > > look at that subsystem, and it seems you can execute usermode helpers > > > > from > > > > atomic sections with help of UMH_NO_WAIT flag. > > > > > > > > Then I checked where this flag is used and it turns out its from the > > > > mce_work_trigger function in x86/kernel/cpu/mcheck/dev-mcelog.c which > > > > can be > > > > called infact from an interrupt context (mce_notify_irq). > > > > > > > > Is this the usecase you remember causing this weird transitions to > > > > userspace? > > > > > > But this case still looks like it uses work queues, it just doesn't > > > wait for the result. > > > > > > I'll have to look at the code from what it looked like back in 2011, to > > > see if there was an actual issue here back then. > > > > Good point Steve. So I guess in the current kernel sources, there's no code > > that uses UMH in IRQ context AFAICT. I'll go through the google group thread > > Paul pointed as well to study the history of the problem a bit more. > > Me too. Good discussion we had thanks to you, Joel.
No problem, thanks for the patch in the first place which triggered this discussion. For whatever its worth, I made some notes of what I understood from reading the code and old posts because I was sure I would otherwise forget everything: http://www.joelfernandes.org/linuxinternals/2018/06/15/rcu-dynticks.html Feel free to comment on that post directly (or here) if you feel something is grossly wrong. Again thank you and everyone for the discussion! ;-) - Joel
