On Wed, Jul 18, 2012 at 01:20:29PM +0300, Michael S. Tsirkin wrote: > On Wed, Jul 18, 2012 at 09:27:42AM +0300, Gleb Natapov wrote: > > On Tue, Jul 17, 2012 at 07:14:52PM +0300, Michael S. Tsirkin wrote: > > > > _Seems_ racy, or _is_ racy? Please identify the race. > > > > > > Look at this: > > > > > > static inline int kvm_irq_line_state(unsigned long *irq_state, > > > int irq_source_id, int level) > > > { > > > /* Logical OR for level trig interrupt */ > > > if (level) > > > set_bit(irq_source_id, irq_state); > > > else > > > clear_bit(irq_source_id, irq_state); > > > > > > return !!(*irq_state); > > > } > > > > > > > > > Now: > > > If other CPU changes some other bit after the atomic change, > > > it looks like !!(*irq_state) might return a stale value. > > > > > > CPU 0 clears bit 0. CPU 1 sets bit 1. CPU 1 sets level to 1. > > > If CPU 0 sees a stale value now it will return 0 here > > > and interrupt will get cleared. > > > > > This will hardly happen on x86 especially since bit is set with > > serialized instruction. > > Probably. But it does make me a bit uneasy. Why don't we pass > irq_source_id to kvm_pic_set_irq/kvm_ioapic_set_irq, and move > kvm_irq_line_state to under pic_lock/ioapic_lock? We can then use > __set_bit/__clear_bit in kvm_irq_line_state, making the ordering simpler > and saving an atomic op in the process. > With my patch I do not see why we can't change them to unlocked variant without moving them anywhere. The only requirement is to not use RMW sequence to set/clear bits. The ordering of setting does not matter. The ordering of reading is. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html