ACPI / processor: prevent cpu from becoming online

Even if acpi_processor_handle_eject() offlines cpu, there is a chance
to online the cpu after that. So the patch closes the window by using
get/put_online_cpus().

Why does the patch change _cpu_up() logic?

The patch cares the race of hot-remove cpu and _cpu_up(). If the patch
does not change it, there is the following race.

hot-remove cpu                         |  _cpu_up()
------------------------------------- ------------------------------------
call acpi_processor_handle_eject()     |
     call cpu_down()                   |
     call get_online_cpus()            |
                                       | call cpu_hotplug_begin() and stop here
     call arch_unregister_cpu()        |
     call acpi_unmap_lsapic()          |
     call put_online_cpus()            |
                                       | start and continue _cpu_up()
     return acpi_processor_remove()    |
continue hot-remove the cpu            |

So _cpu_up() can continue to itself. And hot-remove cpu can also continue
itself. If the patch changes _cpu_up() logic, the race disappears as below:

hot-remove cpu                         | _cpu_up()
-----------------------------------------------------------------------
call acpi_processor_handle_eject()     |
     call cpu_down()                   |
     call get_online_cpus()            |
                                       | call cpu_hotplug_begin() and stop here
     call arch_unregister_cpu()        |
     call acpi_unmap_lsapic()          |
          cpu's cpu_present is set     |
          to false by set_cpu_present()|
     call put_online_cpus()            |
                                       | start _cpu_up()
                                       | check cpu_present() and return -EINVAL
     return acpi_processor_remove()    |
continue hot-remove the cpu            |

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Reviewed-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/drivers/acpi/processor_driver.c b/drivers/acpi/processor_driver.c
index bd4e5dca3ff7..a4352b88a331 100644
--- a/drivers/acpi/processor_driver.c
+++ b/drivers/acpi/processor_driver.c
@@ -852,8 +852,22 @@ static int acpi_processor_handle_eject(struct acpi_processor *pr)
         if (cpu_online(pr->id))
                 cpu_down(pr->id);
 
+        get_online_cpus();
+        /*
+         * The cpu might become online again at this point. So we check whether
+         * the cpu has been onlined or not. If the cpu became online, it means
+         * that someone wants to use the cpu. So acpi_processor_handle_eject()
+         * returns -EAGAIN.
+         */
+        if (unlikely(cpu_online(pr->id))) {
+                put_online_cpus();
+                pr_warn("Failed to remove CPU %d, because other task "
+                        "brought the CPU back online\n", pr->id);
+                return -EAGAIN;
+        }
         arch_unregister_cpu(pr->id);
         acpi_unmap_lsapic(pr->id);
+        put_online_cpus();
         return (0);
 }
 #else