[Xen-devel] [PATCH RFC V2 2/5] stop_machine: make stop_machine safe and efficient to call early

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

Make stop_machine() safe to call early in boot, before SMP has been
set up, by simply calling the callback function directly if there's
only one CPU online.

[ Fixes from AKPM:
   - add comment
   - local_irq_flags, not save_flags
   - also call hard_irq_disable() for systems which need it

  Tejun suggested using an explicit flag rather than just looking at
  the online cpu count. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/stop_machine.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index ba5070c..9c59d9e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -41,6 +41,7 @@ struct cpu_stopper {
 };
 
 static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
+static bool stop_machine_initialized = false;
 
 static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int 
nr_todo)
 {
@@ -386,6 +387,8 @@ static int __init cpu_stop_init(void)
        cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_ONLINE, bcpu);
        register_cpu_notifier(&cpu_stop_cpu_notifier);
 
+       stop_machine_initialized = true;
+
        return 0;
 }
 early_initcall(cpu_stop_init);
@@ -485,6 +488,24 @@ int __stop_machine(int (*fn)(void *), void *data, const 
struct cpumask *cpus)
                                            .num_threads = num_online_cpus(),
                                            .active_cpus = cpus };
 
+       if (!stop_machine_initialized) {
+               /*
+                * Handle the case where stop_machine() is called early in boot
+                * before SMP startup.
+                */
+               unsigned long flags;
+               int ret;
+
+               WARN_ON_ONCE(smdata.num_threads != 1);
+
+               local_irq_save(flags);
+               hard_irq_disable();
+               ret = (*fn)(data);
+               local_irq_restore(flags);
+
+               return ret;
+       }
+
        /* Set the initial state and stop all online cpus. */
        set_state(&smdata, STOPMACHINE_PREPARE);
        return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
-- 
1.7.6.2


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