Hello, Oleg. On Sat, Nov 21, 2015 at 07:11:48PM +0100, Oleg Nesterov wrote: > stop_two_cpus() and stop_cpus() use stop_cpus_lock to avoid the deadlock, > we need to ensure that the stopper functions can't be queued "backwards" > from one another. This doesn't look nice; if we use lglock then we do not > really need stopper->lock, cpu_stop_queue_work() could use lg_local_lock() > under local_irq_save().
Yeah, removing stopper->lock would be nice. > OTOH it would be even better to avoid lglock in stop_machine.c and remove > lg_double_lock(). This patch adds "bool stop_cpus_in_progress" set/cleared > by queue_stop_cpus_work(), and changes cpu_stop_queue_two_works() to busy > wait until it is cleared. > > queue_stop_cpus_work() sets stop_cpus_in_progress = T lockless, but after > it queues a work on CPU1 it must be visible to stop_two_cpus(CPU1, CPU2) > which checks it under the same lock. And since stop_two_cpus() holds the > 2nd lock too, queue_stop_cpus_work() can not clear stop_cpus_in_progress > if it is also going to queue a work on CPU2, it needs to take that 2nd > lock to do this. Isn't this a lot more subtler than the other direction? Unless there's a clear performance advantage to removing stopper->lock, using lglock for both stop_two and stop_machine seems like an easier-to-follow approach to me. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
