Re: [PATCH -tip/cpus4096-v2] cpumask: fix cpumask of call_function_data

[Rusty Russell <rusty@rustcorp.com.au>]

On Friday 24 October 2008 15:47:20 Hiroshi Shimamoto wrote:
> From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
>
> The following assignment in smp_call_function_many() may cause unexpected
> behavior, when !CPUMASK_OFFSTACK.
> 	data->cpumask = allbutself;
>
> Because it copys pointer of stack and the value will be modified after
> exit from smp_call_function_many().

Good catch!

> The type of cpumask field of call_function_data structure should be
> cpumask_var_t and an operation to assign is needed.

This makes the lifetime rules dependent on the config option, which is
complicated.

Your insight into this issue is appreciated: this code is not simple!

How's this version instead?  It puts the cpumask at the end of the kmalloc,
and falls back to smp_call_function_single instead of doing obscure
quiescing stuff.

(Compiles, untested).

Thanks!
Rusty.

diff -r 60e2190a18cd kernel/smp.c
--- a/kernel/smp.c	Fri Oct 24 20:22:52 2008 +1100
+++ b/kernel/smp.c	Fri Oct 24 22:02:59 2008 +1100
@@ -24,8 +24,8 @@ struct call_function_data {
 	struct call_single_data csd;
 	spinlock_t lock;
 	unsigned int refs;
-	struct cpumask *cpumask;
 	struct rcu_head rcu_head;
+	unsigned long cpumask_bits[];
 };
 
 struct call_single_queue {
@@ -109,13 +109,13 @@ void generic_smp_call_function_interrupt
 	list_for_each_entry_rcu(data, &call_function_queue, csd.list) {
 		int refs;
 
-		if (!cpumask_test_cpu(cpu, data->cpumask))
+		if (!cpumask_test_cpu(cpu, to_cpumask(data->cpumask_bits)))
 			continue;
 
 		data->csd.func(data->csd.info);
 
 		spin_lock(&data->lock);
-		cpumask_clear_cpu(cpu, data->cpumask);
+		cpumask_clear_cpu(cpu, to_cpumask(data->cpumask_bits));
 		WARN_ON(data->refs == 0);
 		data->refs--;
 		refs = data->refs;
@@ -265,42 +265,6 @@ void __smp_call_function_single(int cpu,
 	generic_exec_single(cpu, data);
 }
 
-/* Dummy function */
-static void quiesce_dummy(void *unused)
-{
-}
-
-/*
- * Ensure stack based data used in call function mask is safe to free.
- *
- * This is needed by smp_call_function_many when using on-stack data, because
- * a single call function queue is shared by all CPUs, and any CPU may pick up
- * the data item on the queue at any time before it is deleted. So we need to
- * ensure that all CPUs have transitioned through a quiescent state after
- * this call.
- *
- * This is a very slow function, implemented by sending synchronous IPIs to
- * all possible CPUs. For this reason, we have to alloc data rather than use
- * stack based data even in the case of synchronous calls. The stack based
- * data is then just used for deadlock/oom fallback which will be very rare.
- *
- * If a faster scheme can be made, we could go back to preferring stack based
- * data -- the data allocation/free is non-zero cost.
- */
-static void smp_call_function_mask_quiesce_stack(const struct cpumask *mask)
-{
-	struct call_single_data data;
-	int cpu;
-
-	data.func = quiesce_dummy;
-	data.info = NULL;
-
-	for_each_cpu(cpu, mask) {
-		data.flags = CSD_FLAG_WAIT;
-		generic_exec_single(cpu, &data);
-	}
-}
-
 /**
  * smp_call_function_many(): Run a function on a set of other CPUs.
  * @mask: The set of cpus to run on (only runs on online subset).
@@ -320,73 +284,59 @@ void smp_call_function_many(const struct
 			    void (*func)(void *), void *info,
 			    bool wait)
 {
-	struct call_function_data d;
-	struct call_function_data *data = NULL;
-	cpumask_var_t allbutself;
+	struct call_function_data *data;
 	unsigned long flags;
-	int cpu, num_cpus;
-	int slowpath = 0;
+	int cpu, next_cpu;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON(irqs_disabled());
 
-	if (!alloc_cpumask_var(&allbutself, GFP_ATOMIC)) {
+	/* So, what's a CPU they want?  Ignoring this one. */
+	cpu = cpumask_first_and(mask, cpu_online_mask);
+	if (cpu == smp_processor_id())
+		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
+	/* Nothing?  We're done. */
+	if (cpu >= nr_cpu_ids)
+		return;
+
+	/* Do we have another CPU which isn't us? */
+	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
+	if (cpu == smp_processor_id())
+		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
+
+	/* Nope!  Fastpath: do that cpu by itself. */
+	if (next_cpu >= nr_cpu_ids)
+		smp_call_function_single(cpu, func, info, wait);
+
+	data = kmalloc(sizeof(*data) + cpumask_size(), GFP_ATOMIC);
+	if (unlikely(!data)) {
 		/* Slow path. */
 		for_each_online_cpu(cpu) {
 			if (cpumask_test_cpu(cpu, mask))
 				smp_call_function_single(cpu, func, info, wait);
 		}
-		return;
-	}
-	cpumask_and(allbutself, cpu_online_mask, mask);
-	cpumask_clear_cpu(smp_processor_id(), allbutself);
-	num_cpus = cpumask_weight(allbutself);
-
-	/*
-	 * If zero CPUs, return. If just a single CPU, turn this request
-	 * into a targetted single call instead since it's faster.
-	 */
-	if (!num_cpus)
-		return;
-	else if (num_cpus == 1) {
-		cpu = cpumask_first(allbutself);
-		smp_call_function_single(cpu, func, info, wait);
-		goto out;
-	}
-
-	data = kmalloc(sizeof(*data), GFP_ATOMIC);
-	if (data) {
-		data->csd.flags = CSD_FLAG_ALLOC;
-		if (wait)
-			data->csd.flags |= CSD_FLAG_WAIT;
-	} else {
-		data = &d;
-		data->csd.flags = CSD_FLAG_WAIT;
-		wait = 1;
-		slowpath = 1;
+		return;		
 	}
 
 	spin_lock_init(&data->lock);
+	data->csd.flags = CSD_FLAG_ALLOC;
+	if (wait)
+		data->csd.flags |= CSD_FLAG_WAIT;
 	data->csd.func = func;
 	data->csd.info = info;
-	data->refs = num_cpus;
-	data->cpumask = allbutself;
+	cpumask_and(to_cpumask(data->cpumask_bits), mask, cpu_online_mask);
+	data->refs = cpumask_weight(to_cpumask(data->cpumask_bits));
 
 	spin_lock_irqsave(&call_function_lock, flags);
 	list_add_tail_rcu(&data->csd.list, &call_function_queue);
 	spin_unlock_irqrestore(&call_function_lock, flags);
 
 	/* Send a message to all CPUs in the map */
-	arch_send_call_function_ipi((cpumask_t)*allbutself);
+	arch_send_call_function_ipi(*to_cpumask(data->cpumask_bits));
 
 	/* optionally wait for the CPUs to complete */
-	if (wait) {
+	if (wait)
 		csd_flag_wait(&data->csd);
-		if (unlikely(slowpath))
-			smp_call_function_mask_quiesce_stack(allbutself);
-	}
-out:
-	free_cpumask_var(allbutself);
 }
 EXPORT_SYMBOL(smp_call_function_many);