On 19/01/15 17:48, Russell King - ARM Linux wrote: > On Tue, Jan 13, 2015 at 04:35:31PM +0000, Daniel Thompson wrote: >> +/* >> + * This handler is called *unconditionally* from the default NMI/FIQ >> + * handler. The irq may not be anything to do with us so the main >> + * job of this function is to figure out if the irq passed in is ours >> + * or not. >> + */ >> +void cpu_pmu_handle_fiq(int irq) >> +{ >> + int cpu = smp_processor_id(); > > This can be either debug_smp_processor_id() or raw_smp_processor_id(). > raw_smp_processor_id() is fine from FIQ contexts, as seems to be > debug_smp_processor_id(), but only because we guarantee that > irqs_disabled() in there will be true.
Curiously I was looking at exactly this yesterday (because I was intrigued why the NMI-safe bits of kgdb use raw_smp_processor_id() but the x86 arch_trigger_all_cpu_backtrace() implementation uses smp_processor_id()). Given the comments make clear smp_processor_id() is the preferred variant except for false positives I concluded I would continue with smp_processor_id() for any code I write hanging off the default FIQ handler. No objections? >> + >> + if (irq != get_cpu_var(cpu_pmu_irqs)) >> + return; > > get_cpu_var() needs put_cpu_var() to undo its effects. get_cpu_var() > calls preempt_disable(), which calls into lockdep... I think we > determined that was fine last time we went digging? Yes. We reviewed lockdep from the point-of-view of RCU and found that lockdep disabled most of itself when in_nmi() is true. > put_cpu_var() > would call preempt_enable() which I'd hope would be safe in FIQ/NMI > contexts? Yes. preempt_count_add/sub form part of the work done by nmi_enter() and nmi_exit(). However this code gets no benefit from calling get_cpu_var(). I think it would be better to switch it to this_cpu_ptr. >> + >> + (void)armpmu_dispatch_irq(irq, >> + get_cpu_ptr(&cpu_pmu->hw_events->percpu_pmu)); > > Again, get_cpu_xxx() needs to be balanced with a put_cpu_xxx(). > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/