On 01/21/20 18:09, Russell King - ARM Linux admin wrote:
On Tue, Jan 21, 2020 at 05:47:52PM +0000, Qais Yousef wrote:
quoted
On 01/21/20 17:03, Russell King - ARM Linux admin wrote:
quoted
On Mon, Nov 25, 2019 at 11:27:41AM +0000, Qais Yousef wrote:
quoted
+void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
+{
+ unsigned int cpu;
+
+ if (!cpu_online(primary_cpu)) {
+ pr_info("Attempting to shutdodwn nonboot cpus while boot cpu is offline!\n");
+ cpu_online(primary_cpu);
Eh, that should be cpu_up(primary_cpu)!
Which I have to say I'm not if is the right thing to do.
migrate_to_reboot_cpu() picks the first online cpu if reboot_cpu (assumed 0) is
offline
migrate_to_reboot_cpu():
225 /* Make certain the cpu I'm about to reboot on is online */
226 if (!cpu_online(cpu))
227 cpu = cpumask_first(cpu_online_mask);
quoted
quoted
+ }
+
+ for_each_present_cpu(cpu) {
+ if (cpu == primary_cpu)
+ continue;
+ if (cpu_online(cpu))
+ cpu_down(cpu);
+ }
How does this avoid racing with userspace attempting to restart CPUs
that have already been taken down by this function?
This is meant to be called from machine_shutdown() only.
But you've got a point.
The previous logic that used disable_nonboot_cpus(), which in turn called
freeze_secondary_cpus() didn't hold hotplug lock. So I assumed the higher level
logic of machine_shutdown() ensures that hotplug lock is held to synchronize
with potential other hotplug operations.
freeze_secondary_cpus() takes the CPU maps lock while it takes CPUs
down, and then disables cpu hotplug by incrementing
cpu_hotplug_disabled. Incrementing that prevents cpu_up() and
cpu_down() being used, thereby preventing userspace from changing the
online state of any CPU in the system.
I see. Sorry I missed the CPU maps lock.
Yes this makes sense and should work here too.
Thanks for the help.
Thomas, I'll wait for your comment on this and potentially other patches before
sending v3.
Thanks
--
Qais Yousef
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