linux-kernel - Re: AW: [PATCH RT RFC v4 1/8] add generalized priority-inheritance interface

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Date:	Tue, 19 Aug 2008 04:34:37 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Matthias Behr <linux@...ehr.de>
CC:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	gregory.haskins@...il.com, mingo@...e.hu, David.Holmes@....com,
	jkacur@...il.com, paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
	tglx@...utronix.de, rostedt@...dmis.org
Subject: Re: AW: [PATCH RT RFC v4 1/8] add generalized priority-inheritance
 interface

Hi Matthias,

Matthias Behr wrote:
> Hi Greg,
>
> I got a few review comments/questions. Pls see below.
>
> Best Regards,
> Matthias
>
> P.S. I'm a kernel newbie so don't hesitate to tell me if I'm wrong ;-)
>
>   
>> +/**
>> + * pi_sink_init - initialize a pi_sink before use
>> + * @sink: a sink context
>> + * @ops: pointer to an pi_sink_ops structure
>> + */
>> +static inline void
>> +pi_sink_init(struct pi_sink *sink, struct pi_sink_ops *ops)
>> +{
>> +	atomic_set(&sink->refs, 0);
>> +	sink->ops = ops;
>> +}
>>     
>
> Shouldn't ops be tested for 0 here? (ASSERT/BUG_ON/...) (get's dereferenced later quite often in the form "if (sink->ops->...)".
>   

This is a good idea.  I will add this.

>   
>> +/**
>> + * pi_sink_put - down the reference count, freeing the sink if 0
>> + * @node: the node context
>> + * @flags: optional flags to modify behavior.  Reserved, must be 0.
>> + *
>> + * Returns: none
>> + */
>> +static inline void
>> +pi_sink_put(struct pi_sink *sink, unsigned int flags)
>> +{
>> +	if (atomic_dec_and_test(&sink->refs)) {
>> +		if (sink->ops->free)
>> +			sink->ops->free(sink, flags);
>> +	}
>> +}
>>     
>
> Shouldn't the atomic/locked part cover the ...->free(...) as well?

Actually, it already does.  The free can only be called by the last 
reference dropping the ref-count.


>  A pi_get right after the atomic_dec_and_test but before the free() could lead to a free() with refs>0?
>   

A pi_get() after the ref could have already dropped to zero is broken at 
a higher layer.  E.g. the caller of pi_get() has to ensure that there 
are no races against the reference dropping to begin with.  This is the 
same as any reference-counted object (for instance, see get_task_struct()).

Thanks for the review, Matthias!

Regards,
-Greg



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