linux-kernel - Re: [patch,v2] bdi: add a user-tunable cpu

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Message-ID: <x49liddq3o0.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 04 Dec 2012 09:42:55 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Zach Brown <zab@...hat.com>
Subject: Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher threads

Dave Chinner <david@...morbit.com> 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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