Am 26.11.2014 um 16:37 schrieb Michael S. Tsirkin: > On Wed, Nov 26, 2014 at 04:30:32PM +0100, Christian Borntraeger wrote: >> Am 26.11.2014 um 16:17 schrieb Michael S. Tsirkin: >>> On Wed, Nov 26, 2014 at 11:05:04AM +0100, David Hildenbrand wrote: >>>>> What's the path you are trying to debug? >>>> >>>> Well, we had a problem where we held a spin_lock and called >>>> copy_(from|to)_user(). We experienced very random deadlocks that took some guy >>>> almost a week to debug. The simple might_sleep() check would have showed this >>>> error immediately. >>> >> >>> This must have been a very old kernel. >>> A modern kernel will return an error from copy_to_user. >> >> I disagree. copy_to_user will not return while holding a spinlock, because it does not know! How should it? >> See: spin_lock will call preempt_disable, but thats a no-op for a non-preempt kernel. So the mere fact that we hold a spin_lock is not known by any user access function. (or others). No? >> >> Christian >> >> > > Well might_sleep() merely checks preempt count and irqs_disabled too. > If you want debugging things to trigger, you need to enable > a bunch of config options. That's not new. You miss the point of the whole thread: The problem is that even with debug options enabled, holding a spinlock would not trigger a bug on copy_to_user. So the problem is not the good path, the problem is that a debugging aid for detecting a broken case was lost. Even with all kernel debugging enabled. That is because CONFIG_DEBUG_ATOMIC_SLEEP selects PREEMPT_COUNT. That means: spin_lock will then be considered as in_atomic and no message comes. Without CONFIG_DEBUG_ATOMIC_SLEEP spin_lock will not touch the preempt_count but we also dont see a message because might_fault is now a nop I understand that you dont like Davids changes due to other side effects that you have mentioned. So lets focus on how we can fix the debug option. Ok? Christian -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
