On Sun, Jul 26, 2020 at 02:14:34PM +1000, Nicholas Piggin wrote: > Excerpts from Peter Zijlstra's message of July 26, 2020 6:26 am: > > Which is 'funny' when it interleaves like: > > > > local_irq_disable(); > > ... > > local_irq_enable() > > trace_hardirqs_on(); > > <NMI/> > > raw_local_irq_enable(); > > > > Because then it will undo the trace_hardirqs_on() we just did. With the > > result that both tracing and lockdep will see a hardirqs-disable without > > a matching enable, while the hardware state is enabled. > > Seems like an arch problem -- why not disable if it was enabled only? > I guess the local_irq tracing calls are a mess so maybe they copied > those. Because, as I wrote earlier, then we can miss updating software state. So your proposal has: raw_local_irq_disable() <NMI> if (!arch_irqs_disabled(regs->flags) // false trace_hardirqs_off(); // tracing/lockdep still think IRQs are enabled // hardware IRQ state is disabled. With the current code we have: local_irq_enable() trace_hardirqs_on(); <NMI> trace_hardirqs_off(); ... if (!arch_irqs_disabled(regs->flags)) // false trace_hardirqs_on(); </NMI> // and now the NMI disabled software state again // while we're about to enable the hardware state raw_local_irq_enable(); > > Which is exactly the state Alexey seems to have ran into. > > No his was what I said, the interruptee's trace_hardirqs_on() in > local_irq_enable getting lost because the NMI's local_irq_disable > always disables, but the enable doesn't re-enable. That's _exactly_ the case above. It doesn't re-enable because hardirqs are actually still disabled. You _cannot_ rely on hardirq state for NMIs, that'll get you wrong state. > It's all just weird asymmetrical special case hacks AFAIKS, the > code should just be symmetric and lockdep handle it's own weirdness. It's for non-maskable exceptions/interrupts, because there the hardware and software state changes non-atomically. For maskable interrupts doing the software state transitions inside the disabled region makes perfect sense, because that keeps it atomic.
