On Sat, Jan 07, 2012 at 08:52:01AM -0800, Paul E. McKenney wrote: > On Thu, Jan 05, 2012 at 04:17:39PM +0000, Mel Gorman wrote: > > On Thu, Jan 05, 2012 at 02:40:11PM +0000, Russell King - ARM Linux wrote: > > > On Thu, Jan 05, 2012 at 02:20:17PM +0000, Mel Gorman wrote: > > [ . . . ] > > > > I've been chasing that patch and getting no replies what so > > > ever from folk like Peter, Thomas and Ingo. > > > > > > The problem affects all IPI-raising functions, which mask with > > > cpu_online_mask directly. > > > > Actually, in one sense I'm glad to hear it because from my brief > > poking around, I was having trouble understanding why we were always > > safe from sending IPIs to CPUs in the process of being offlined. > > The trick is to disable preemption (not interrupts!) across the IPI, which > prevents CPU-hotplug's stop_machine() from running. You also have to > have checked that the CPU is online within this same preemption-disabled > section of code. This means that the outgoing CPU has to accept IPIs > even after its CPU_DOWN_PREPARE notifier has been called -- right up > to the stop_machine() call to take_cpu_down(). Of course, another trick is to hold the CPU-hotplug lock across the IPI, but this is quite a bit more heavy-weight than disabling preemption. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>