Andy Lutomirski <l...@kernel.org> writes: > 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.
I thought about that, but then ended up with the conclusion that RCU might be unhappy, but my conclusion might be fundamentally wrong. Paul? Thanks, tglx