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SubjectRe: [PATCH -tip/cpus4096-v2] cpumask: fix cpumask of call_function_data
Date
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);





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