stop_machine: implement stop_machine_from_inactive_cpu()

Currently, mtrr wants stop_machine functionality while a CPU is being
brought up.  As stop_machine() requires the calling CPU to be active,
mtrr implements its own stop_machine using stop_one_cpu() on each
online CPU.  This doesn't only unnecessarily duplicate complex logic
but also introduces a possibility of deadlock when it races against
the generic stop_machine().

This patch implements stop_machine_from_inactive_cpu() to serve such
use cases.  Its functionality is basically the same as stop_machine();
however, it should be called from a CPU which isn't active and doesn't
depend on working scheduling on the calling CPU.

This is achieved by using busy loops for synchronization and
open-coding stop_cpus queuing and waiting with direct invocation of
fn() for local CPU inbetween.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20110623182056.982526827@sbsiddha-MOBL3.sc.intel.com
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 61 insertions, 1 deletions

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 4c89ee9fc56b..e8f05b14cd43 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -439,8 +439,15 @@ static int stop_machine_cpu_stop(void *data)
         struct stop_machine_data *smdata = data;
         enum stopmachine_state curstate = STOPMACHINE_NONE;
         int cpu = smp_processor_id(), err = 0;
+        unsigned long flags;
         bool is_active;
 
+        /*
+         * When called from stop_machine_from_inactive_cpu(), irq might
+         * already be disabled.  Save the state and restore it on exit.
+         */
+        local_save_flags(flags);
+
         if (!smdata->active_cpus)
                 is_active = cpu == cpumask_first(cpu_online_mask);
         else
@@ -468,7 +475,7 @@ static int stop_machine_cpu_stop(void *data)
                 }
         } while (curstate != STOPMACHINE_EXIT);
 
-        local_irq_enable();
+        local_irq_restore(flags);
         return err;
 }
 
@@ -495,4 +502,57 @@ int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 }
 EXPORT_SYMBOL_GPL(stop_machine);
 
+/**
+ * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
+ * @fn: the function to run
+ * @data: the data ptr for the @fn()
+ * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
+ *
+ * This is identical to stop_machine() but can be called from a CPU which
+ * is not active.  The local CPU is in the process of hotplug (so no other
+ * CPU hotplug can start) and not marked active and doesn't have enough
+ * context to sleep.
+ *
+ * This function provides stop_machine() functionality for such state by
+ * using busy-wait for synchronization and executing @fn directly for local
+ * CPU.
+ *
+ * CONTEXT:
+ * Local CPU is inactive.  Temporarily stops all active CPUs.
+ *
+ * RETURNS:
+ * 0 if all executions of @fn returned 0, any non zero return value if any
+ * returned non zero.
+ */
+int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
+                                  const struct cpumask *cpus)
+{
+        struct stop_machine_data smdata = { .fn = fn, .data = data,
+                                            .active_cpus = cpus };
+        struct cpu_stop_done done;
+        int ret;
+
+        /* Local CPU must be inactive and CPU hotplug in progress. */
+        BUG_ON(cpu_active(raw_smp_processor_id()));
+        smdata.num_threads = num_active_cpus() + 1;     /* +1 for local */
+
+        /* No proper task established and can't sleep - busy wait for lock. */
+        while (!mutex_trylock(&stop_cpus_mutex))
+                cpu_relax();
+
+        /* Schedule work on other CPUs and execute directly for local CPU */
+        set_state(&smdata, STOPMACHINE_PREPARE);
+        cpu_stop_init_done(&done, num_active_cpus());
+        queue_stop_cpus_work(cpu_active_mask, stop_machine_cpu_stop, &smdata,
+                             &done);
+        ret = stop_machine_cpu_stop(&smdata);
+
+        /* Busy wait for completion. */
+        while (!completion_done(&done.completion))
+                cpu_relax();
+
+        mutex_unlock(&stop_cpus_mutex);
+        return ret ?: done.ret;
+}
+
 #endif  /* CONFIG_STOP_MACHINE */