linux-kernel - Re: RFC: Boiler plate functions for ida / idr allocation?

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Date:	Wed, 13 Jul 2011 14:17:08 +0100
From:	Jonathan Cameron <jic23@....ac.uk>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
CC:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: RFC: Boiler plate functions for ida / idr allocation?

On 07/13/11 14:14, Stefan Richter wrote:
> On Jul 13 Jonathan Cameron wrote:
>> Taking ida's first, how about the following patch?  I'm not at
>> all attached to the form it takes, merely to cutting out on the
>> cut and paste.
> 
> Not a big-picture opinion here whether this is a good thing; only some
> small comments on side issues:
> 
> [...]
>> The other thing this highlights is that I suspect quite a few are protected by
>> spin locks when a mutex would be fine. Hence that might be worth tidying up first.
> 
> It seems to be the other way around in this case:  Why use a mutex if a
> spinlock is fine?
> 
> [...]
>> --- a/drivers/misc/cb710/core.c
>> +++ b/drivers/misc/cb710/core.c
>> @@ -254,18 +254,9 @@ static int __devinit cb710_probe(struct pci_dev *pdev,
>>  	if (err)
>>  		return err;
>>  
>> -	do {
>> -		if (!ida_pre_get(&cb710_ida, GFP_KERNEL))
>> -			return -ENOMEM;
>> -
>> -		spin_lock_irqsave(&cb710_ida_lock, flags);
>> -		err = ida_get_new(&cb710_ida, &chip->platform_id);
>> -		spin_unlock_irqrestore(&cb710_ida_lock, flags);
>> -
>> -		if (err && err != -EAGAIN)
>> -			return err;
>> -	} while (err);
>> -
>> +	err = ida_get_id(&cb710_ida_lock, &cb710_ida, &chip->platform_id);
>> +	if (err)
>> +		return err;
>>  
>>  	dev_info(&pdev->dev, "id %d, IO 0x%p, IRQ %d\n",
>>  		chip->platform_id, chip->iobase, pdev->irq);
> 
> To balance this change to cb710_probe, also switch from spin_lock_irqsave/
> spin_unlock_irqrestore to spin_lock/spin_unlock in cb710_remove_one for
> clarity.
Good point. I'd actually suggest adding a paired release_id function with similar
semantics to the get function just to keep things consistent.
> 
> [...]
>> --- a/lib/idr.c
>> +++ b/lib/idr.c
>> @@ -939,3 +939,23 @@ void ida_init(struct ida *ida)
>>  
>>  }
>>  EXPORT_SYMBOL(ida_init);
>> +
>> +int ida_get_id(spinlock_t *lock, struct ida *ida, int *val)
>> +{
>> +	int ret = 0;
>> +ida_again:
>> +	if (unlikely(ida_pre_get(ida, GFP_KERNEL) == 0))
>> +		return -ENOMEM;
>> +
>> +	spin_lock(lock);
>> +	ret = ida_get_new(ida, val);
>> +	spin_unlock(lock);
>> +
>> +	if (unlikely (ret == -EAGAIN))
>> +		goto ida_again;
>> +	else if (likely(!ret))
>> +		*val = *val & MAX_ID_MASK;
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(ida_get_id);
> 
> A new exported function (in lib/ even) should come with a kerneldoc comment
> of course.  Here it is among else noteworthy that the caller must provide
> GFP_KERNEL allocations capable context and that @lock cannot be shared
> with users in IRQ or softIRQ contexts.
Of course, I was just being lazy for an RFC ;)  Is suspect any formal submission
will need a few iterations before everyone is happy.
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