[patch v3 1/2] stop_machine: enable __stop_machine() to be called from the cpu online path

[Suresh Siddha]

Currently stop machine infrastructure can be called only from a cpu that is
online. But for !CONFIG_SMP, we do allow calling __stop_machine() before the
cpu is online.

x86 for example requires stop machine infrastructure in the cpu online path
and currently implements its own stop machine (using stop_one_cpu_nowait())
for MTRR initialization in the cpu online path.

Enhance the __stop_machine() so that it can be called before the cpu
is onlined. This will pave the way for code consolidation and address potential
deadlocks caused by multiple mechanisms of doing system wide rendezvous.

This will also address the behavioral differences of __stop_machine()
between SMP and UP builds.

Also mark __stop_cpus() to be static, no one else uses it.

Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
Cc: stable@xxxxxxxxxx    # v2.6.35+
---
 kernel/stop_machine.c |   58 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 49 insertions(+), 9 deletions(-)

Index: linux-2.6-tip/kernel/stop_machine.c
===================================================================
--- linux-2.6-tip.orig/kernel/stop_machine.c
+++ linux-2.6-tip/kernel/stop_machine.c
@@ -28,6 +28,7 @@
 struct cpu_stop_done {
 	atomic_t		nr_todo;	/* nr left to execute */
 	bool			executed;	/* actually executed? */
+	bool			offline_ctxt;	/* stop_cpu from offline ctxt */
 	int			ret;		/* collected return value */
 	struct completion	completion;	/* fired if nr_todo reaches 0 */
 };
@@ -47,15 +48,32 @@ static void cpu_stop_init_done(struct cp
 	memset(done, 0, sizeof(*done));
 	atomic_set(&done->nr_todo, nr_todo);
 	init_completion(&done->completion);
+	done->offline_ctxt = !percpu_read(cpu_stopper.enabled);
+}
+
+static inline void cpu_stop_wait_for_completion(struct cpu_stop_done *done)
+{
+	if (!done->offline_ctxt)
+		wait_for_completion(&done->completion);
+	else {
+		/*
+		 * If the calling cpu is not online, then we can't afford to
+		 * sleep, so poll till the work is completed on the target
+		 * cpu's.
+		 */
+		while (atomic_read(&done->nr_todo))
+			cpu_relax();
+	}
 }
 
 /* signal completion unless @done is NULL */
 static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
 {
 	if (done) {
+		bool offline_ctxt = done->offline_ctxt;
 		if (executed)
 			done->executed = true;
-		if (atomic_dec_and_test(&done->nr_todo))
+		if (atomic_dec_and_test(&done->nr_todo) &&  !offline_ctxt)
 			complete(&done->completion);
 	}
 }
@@ -108,7 +126,7 @@ int stop_one_cpu(unsigned int cpu, cpu_s
 
 	cpu_stop_init_done(&done, 1);
 	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), &work);
-	wait_for_completion(&done.completion);
+	cpu_stop_wait_for_completion(&done);
 	return done.executed ? done.ret : -ENOENT;
 }
 
@@ -136,20 +154,24 @@ void stop_one_cpu_nowait(unsigned int cp
 static DEFINE_MUTEX(stop_cpus_mutex);
 static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);
 
+static
 int __stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 {
+	int online = percpu_read(cpu_stopper.enabled);
 	struct cpu_stop_work *work;
 	struct cpu_stop_done done;
+	unsigned int weight = 0;
 	unsigned int cpu;
 
 	/* initialize works and done */
-	for_each_cpu(cpu, cpumask) {
+	for_each_cpu_and(cpu, cpumask, cpu_online_mask) {
 		work = &per_cpu(stop_cpus_work, cpu);
 		work->fn = fn;
 		work->arg = arg;
 		work->done = &done;
+		weight++;
 	}
-	cpu_stop_init_done(&done, cpumask_weight(cpumask));
+	cpu_stop_init_done(&done, weight);
 
 	/*
 	 * Disable preemption while queueing to avoid getting
@@ -157,12 +179,19 @@ int __stop_cpus(const struct cpumask *cp
 	 * to enter @fn which can lead to deadlock.
 	 */
 	preempt_disable();
-	for_each_cpu(cpu, cpumask)
+	for_each_cpu_and(cpu, cpumask, cpu_online_mask)
 		cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
 				    &per_cpu(stop_cpus_work, cpu));
+
+	/*
+	 * This cpu is not yet online. If @fn needs to be run on this
+	 * cpu, run it now.
+	 */
+	if (!online && cpu_isset(smp_processor_id(), *cpumask))
+		fn(arg);
 	preempt_enable();
 
-	wait_for_completion(&done.completion);
+	cpu_stop_wait_for_completion(&done);
 	return done.executed ? done.ret : -ENOENT;
 }
 
@@ -431,6 +460,7 @@ static int stop_machine_cpu_stop(void *d
 	struct stop_machine_data *smdata = data;
 	enum stopmachine_state curstate = STOPMACHINE_NONE;
 	int cpu = smp_processor_id(), err = 0;
+	unsigned long flags = 0;
 	bool is_active;
 
 	if (!smdata->active_cpus)
@@ -446,7 +476,7 @@ static int stop_machine_cpu_stop(void *d
 			curstate = smdata->state;
 			switch (curstate) {
 			case STOPMACHINE_DISABLE_IRQ:
-				local_irq_disable();
+				local_irq_save(flags);
 				hard_irq_disable();
 				break;
 			case STOPMACHINE_RUN:
@@ -460,7 +490,7 @@ static int stop_machine_cpu_stop(void *d
 		}
 	} while (curstate != STOPMACHINE_EXIT);
 
-	local_irq_enable();
+	local_irq_restore(flags);
 	return err;
 }
 
@@ -470,9 +500,19 @@ int __stop_machine(int (*fn)(void *), vo
 					    .num_threads = num_online_cpus(),
 					    .active_cpus = cpus };
 
+	/* Include the calling cpu that might not be online yet. */
+	if (!percpu_read(cpu_stopper.enabled))
+		smdata.num_threads++;
+
 	/* 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);
+
+	if (percpu_read(cpu_stopper.enabled))
+		return stop_cpus(cpu_online_mask, stop_machine_cpu_stop,
+				 &smdata);
+	else
+		return __stop_cpus(cpu_all_mask, stop_machine_cpu_stop,
+				   &smdata);
 }
 
 int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)


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