On Fri, Jun 07, 2013 at 11:55:12PM +0100, Russell King - ARM Linux wrote: > On Fri, Jun 07, 2013 at 04:39:32PM -0600, Stephen Warren wrote: > > On 06/07/2013 04:15 PM, Russell King - ARM Linux wrote: > > > For reboot, the real solution there is not to use software-based > > > reboot, but bring the other cores to a halt (which is what > > > ipi_send_stop is doing) and then issue a hardware reset to the whole > > > system, including the other CPUs. > > > > Ignoring the issues with oops in reboot, I think there's a bug in that > > when hotplug is enabled, smp_kill_cpus() calls platform_cpu_kill(), but > > nothing causes the failing CPU to ever execute smp_ops.cpu_die(). Hence, > > if the implementation of smp_ops.cpu_kill() relies on the target CPU > > having run smp_ops.cpu_die(), then smp_ops.cpu_kill() may not operate > > correctly. > > Well, smp_kill_cpus() was added to get around the kexec problem - > transitioning from one kernel to the next kernel without going through > a hardware reset. Maybe if we take a step back... > > 1. remove smp_kill_cpus() from smp_send_stop(). > 2. remove machine_shutdown() from machine_halt(), machine_power_off() > and machine_restart(). > 3. call smp_send_stop() only from machine_halt(), machine_power_off() and > machine_restart() > 4. require a hardware-based reboot method for all SMP implementations; > using soft_reboot() is not an option. > > This should get us into the situation where we have a reliable method of > halting and rebooting the kernel everywhere, leaving kexec as being the > remaining problem case. > > Currently, for that we effectively do smp_send_stop() followed by > smp_kill_cpus(). The no-op change for kexec there is to allow > smp_kill_cpus() to be called directly from machine_shutdown() - but > I suspect there will still be stuff that's broken with that... > > So the ongoing problem remains - how to deal with kexec in a SMP > environment where it's difficult to reliably take a secondary CPU > offline to a safe place and then be able to restart it into the > next kernel... For kexec, I think it's perfectly reasonable to mandate hardware-based offlining for the secondary cores (hence the half-hearted dependency on HOTPLUG_CPU). In that case, the only guy that has to go down the soft reboot path is the primary CPU which shouldn't be too problematic, right? Supporting sort-reboot of secondaries is a total PITA, even if you have some `safe place' to put them. You still have to synchronise with non-coherent cores so that you know when it's safe to clobber the old image, which requires complex locking algorithms and a prevailing wind. Will -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
