cpumask: don't recommend set_cpus_allowed hack in Documentation/cpu-hotplug.txt

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Ashok Raj <ashok.raj@intel.com>

1 files changed, 17 insertions, 32 deletions

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index 4d4a644b505e..a99d7031cdf9 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -315,41 +315,26 @@ A: The following are what is required for CPU hotplug infrastructure to work
 
 Q: I need to ensure that a particular cpu is not removed when there is some
    work specific to this cpu is in progress.
-A: First switch the current thread context to preferred cpu
+A: There are two ways.  If your code can be run in interrupt context, use
+   smp_call_function_single(), otherwise use work_on_cpu().  Note that
+   work_on_cpu() is slow, and can fail due to out of memory:
 
         int my_func_on_cpu(int cpu)
         {
-                cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
-                int curr_cpu, err = 0;
-
-                saved_mask = current->cpus_allowed;
-                cpu_set(cpu, new_mask);
-                err = set_cpus_allowed(current, new_mask);
-
-                if (err)
-                        return err;
-
-                /*
-                 * If we got scheduled out just after the return from
-                 * set_cpus_allowed() before running the work, this ensures
-                 * we stay locked.
-                 */
-                curr_cpu = get_cpu();
-
-                if (curr_cpu != cpu) {
-                        err = -EAGAIN;
-                        goto ret;
-                } else {
-                        /*
-                         * Do work : But cant sleep, since get_cpu() disables preempt
-                         */
-                }
-                ret:
-                        put_cpu();
-                        set_cpus_allowed(current, saved_mask);
-                        return err;
-                }
-
+                int err;
+                get_online_cpus();
+                if (!cpu_online(cpu))
+                        err = -EINVAL;
+                else
+#if NEEDS_BLOCKING
+                        err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
+#else
+                        smp_call_function_single(cpu, __my_func_on_cpu, &err,
+                                                 true);
+#endif
+                put_online_cpus();
+                return err;
+        }
 
 Q: How do we determine how many CPUs are available for hotplug.
 A: There is no clear spec defined way from ACPI that can give us that