Dave Chinner <david@xxxxxxxxxxxxx> writes:
> On Mon, Dec 03, 2012 at 01:53:39PM -0500, Jeff Moyer wrote:
>> +static ssize_t cpu_list_store(struct device *dev,
>> + struct device_attribute *attr, const char *buf, size_t count)
>> +{
>> + struct backing_dev_info *bdi = dev_get_drvdata(dev);
>> + struct bdi_writeback *wb = &bdi->wb;
>> + cpumask_var_t newmask;
>> + ssize_t ret;
>> + struct task_struct *task;
>> +
>> + if (!alloc_cpumask_var(&newmask, GFP_KERNEL))
>> + return -ENOMEM;
>> +
>> + ret = cpulist_parse(buf, newmask);
>> + if (!ret) {
>> + spin_lock(&bdi->wb_lock);
>> + task = wb->task;
>> + if (task)
>> + get_task_struct(task);
>> + spin_unlock(&bdi->wb_lock);
>> + if (task) {
>> + ret = set_cpus_allowed_ptr(task, newmask);
>> + put_task_struct(task);
>> + }
>
> Why is this set here outside the bdi->flusher_cpumask_mutex?
The cpumask mutex protects updates to bdi->flusher_cpumask, it has
nothing to do with the call to set_cpus_allowed. We are protected from
concurrent calls to cpu_list_store by the sysfs mutex that is taken on
entry. I understand that this is non-obvious, and it wouldn't be wrong
to hold the mutex here. If you'd like me to do that for clarity, that
would be ok with me.
> Also, I'd prefer it named "..._lock" as that is the normal
> convention for such variables. You can tell the type of lock from
> the declaration or the use...
I'm sure I can find counter-examples, but it doesn't really matter to
me. I'll change it.
>> @@ -437,6 +488,14 @@ static int bdi_forker_thread(void *ptr)
>> spin_lock_bh(&bdi->wb_lock);
>> bdi->wb.task = task;
>> spin_unlock_bh(&bdi->wb_lock);
>> + mutex_lock(&bdi->flusher_cpumask_mutex);
>> + ret = set_cpus_allowed_ptr(task,
>> + bdi->flusher_cpumask);
>> + mutex_unlock(&bdi->flusher_cpumask_mutex);
>
> As it is set under the lock here....
It's done under the lock here since we need to keep bdi->flusher_cpumask
from changing during the call to set_cpus_allowed.
Cheers,
Jeff
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