On Wed, Oct 07, 2020 at 10:57:52AM -0400, Mathieu Desnoyers wrote: > ----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote: > > > On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote: > >> diff --git a/kernel/exit.c b/kernel/exit.c > >> index 733e80f334e7..0767a2dbf245 100644 > >> --- a/kernel/exit.c > >> +++ b/kernel/exit.c > >> @@ -475,7 +475,19 @@ static void exit_mm(void) > >> BUG_ON(mm != current->active_mm); > >> /* more a memory barrier than a real lock */ > >> task_lock(current); > >> + /* > >> + * When a thread stops operating on an address space, the loop > >> + * in membarrier_private_expedited() may not observe that > >> + * tsk->mm, and the loop in membarrier_global_expedited() may > >> + * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED > >> + * rq->membarrier_state, so those would not issue an IPI. > >> + * Membarrier requires a memory barrier after accessing > >> + * user-space memory, before clearing tsk->mm or the > >> + * rq->membarrier_state. > >> + */ > >> + smp_mb__after_spinlock(); > >> current->mm = NULL; > >> + membarrier_update_current_mm(NULL); > >> mmap_read_unlock(mm); > >> enter_lazy_tlb(mm, current); > >> task_unlock(current); > > > > This site seems to be lacking in IRQ disabling. As proposed it will > > explode on RT. > > Right, so irq off is needed for accessing this_rq()'s fields safely, > correct ? Yes, but also we're having IRQs disabled on ever other site that mucks with ->mm these days.
