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SubjectRe: [PATCH -tip/cpus4096-v2] cpumask: fix cpumask of call_function_data
Rusty Russell wrote:
> 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.

Okay, I see.

>
> 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).

comments below, not tested.

>
> 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);

I'm not sure,
if next_cpu == smp_processor_id() &&
cpumask_next_and(next_cpu, ...) >= nr_cpu_ids

we can go fastpath, right?

> + if (cpu == smp_processor_id())

so, next_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);

first path, should return here?

> +
> + data = kmalloc(sizeof(*data) + cpumask_size(), GFP_ATOMIC);
> + if (unlikely(!data)) {
> /* Slow path. */
> for_each_online_cpu(cpu) {
> if (cpumask_test_cpu(cpu, mask))

I guess, another issue, should skip when cpu == smp_processor_id().

> 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);

I guess, clear itself is needed.

thanks,
Hiroshi Shimamoto

> + 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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