On Tue, Dec 20, 2011 at 08:05:32PM +0530, Srivatsa S. Bhat wrote: > Sorry but I didn't quite get your point... > No two cpu hotplug operations can race because of the cpu_hotplug lock they > use. Hence, if a cpu online operation begins, it has to succeed or fail > eventually. No other cpu hotplug operation can intervene. Ditto for cpu offline > operations. > > Hence a CPU_UP_PREPARE event *will* be followed by a corresponding > CPU_UP_CANCELED or CPU_ONLINE event for the same cpu. (And we ignore the > CPU_STARTING event that comes in between, on purpose, so as to avoid the race > with cpu_online_mask). Similar is the story for offline operation. > > And if the notifier grabs the spinlock and keeps it locked between these 2 > points of a cpu hotplug operation, it ensures that our br locks will spin, > instead of block till the cpu hotplug operation is complete. Isn't this what > we desired all along? "A non-blocking way to sync br locks with cpu hotplug"? > > Or am I missing something? The standard reason why losing the timeslice while holding a spinlock means deadlocks? CPU1: grabs spinlock CPU[2..n]: tries to grab the same spinlock, spins CPU1: does something blocking, process loses timeslice CPU1: whatever got scheduled there happens to to try and grab the same spinlock and you are stuck. At that point *all* CPUs are spinning on that spinlock and your code that would eventually unlock it has no chance to get any CPU to run on. Having the callback grab and release a spinlock is fine (as long as you don't do anything blocking between these spin_lock/spin_unlock). Having it leave with spinlock held, though, means that the area where you can't block has expanded a whole lot. As I said, brittle... A quick grep through the actual callbacks immediately shows e.g. del_timer_sync() done on CPU_DOWN_PREPARE. And sysfs_remove_group(), which leads to outright mutex_lock(). And sysfs_remove_link() (ditto). And via_cputemp_device_remove() (again, mutex_lock()). And free_irq(). And perf_event_exit_cpu() (mutex_lock()). And... IOW, there are shitloads of deadlocks right there. If your callback's position in the chain is earlier than any of those, you are screwed. No, what I had in mind was different - use the callbacks to maintain a bitmap that would contain a) all CPUs that were online at the moment b) ... and not too much else Updates protected by spinlock; in all cases it gets dropped before the callback returns. br_write_lock() grabs that spinlock and iterates over the set; it *does* leave the spinlock grabbed - that's OK, since all code between br_write_lock() and br_write_unlock() must be non-blocking anyway. br_write_unlock() iterates over the same bitmap (unchanged since br_write_lock()) and finally drops the spinlock. AFAICS, what we want in callback is CPU_DEAD, CPU_DEAD_FROZEN, CPU_UP_CANCELLED, CPU_UP_CANCELLED_FROZEN: grab spinlock remove cpu from bitmap drop spinlock CPU_UP_PREPARE, CPU_UP_PREPARE_FROZEN grab spinlock add cpu to bitmap drop spinlock That ought to keep bitmap close to cpu_online_mask, which is enough for our purposes. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
