On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <t...@linutronix.de> wrote: > > > The pagefault handler cannot use the regular idtentry_enter() because that > invokes rcu_irq_enter() if the pagefault was caused in the kernel. Not a > problem per se, but kernel side page faults can schedule which is not > possible without invoking rcu_irq_exit(). > > Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual > pagefault handling code would be possible, but not pretty either. > > Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only > when RCU is not watching. The conditional RCU enabling is a correctness > issue: A kernel page fault which hits a RCU idle reason can neither > schedule nor is it likely to survive. But avoiding RCU warnings or RCU side > effects is at least increasing the chance for useful debug output. > > The function is also useful for implementing lightweight reschedule IPI and > KVM posted interrupt IPI entry handling later.
Why is this conditional? That is, couldn't we do this for all idtentry_enter() calls instead of just for page faults? Evil things like NMI shouldn't go through this path at all. --Andy
