linux-kernel - Re: [PATCH 2/3] xen-scsiback: One function call less in scsiback_device

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Date:	Wed, 20 Jul 2016 07:10:24 +0200
From:	SF Markus Elfring <elfring@...rs.sourceforge.net>
To:	Jürgen Groß <jgross@...e.com>
Cc:	xen-devel@...ts.xenproject.org, linux-scsi@...r.kernel.org,
	David Vrabel <david.vrabel@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org,
	Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH 2/3] xen-scsiback: One function call less in
 scsiback_device_action() after error detection

>>>> @@ -606,7 +606,7 @@ static void scsiback_device_action(struct vscsibk_pend *pending_req,
>>>>  	tmr = kzalloc(sizeof(struct scsiback_tmr), GFP_KERNEL);
>>>>  	if (!tmr) {
>>>>  		target_put_sess_cmd(se_cmd);
>>>> -		goto err;
>>>> +		goto do_resp;
>>>>  	}
>>>
>>> Hmm, I'm not convinced this is an improvement.
>>>
>>> I'd rather rename the new error label to "put_cmd" and get rid of the
>>> braces in above if statement:
>>>
>>> -	if (!tmr) {
>>> -		target_put_sess_cmd(se_cmd);
>>> -		goto err;
>>> -	}
>>> +	if (!tmr)
>>> +		goto put_cmd;
>>>
>>> and then in the error path:
>>>
>>> -err:
>>> +put_cmd:
>>> +	target_put_sess_cmd(se_cmd);
>>
>> I am unsure on the relevance of this function on such a source position.
>> Would it make sense to move it further down at the end?
> 
> You only want to call it in the first error case (allocation failure).

Thanks for your clarification.

I find that my update suggestion (from Saturday) is still appropriate
in this case.
https://lkml.org/lkml/2016/7/16/172


>>> +free_tmr:
>>> 	kfree(tmr);
>>
>> How do you think about to skip this function call after a memory
>> allocation failure?
> 
> I think this just doesn't matter. If it were a hot path, yes. But trying
> to do micro-optimizations in an error path is just not worth the effort.

Would you like to reduce also the amount of function calls in such special
run-time situations?


> I like a linear error path containing all the needed cleanups best.

I would prefer to keep the discussed single function call within
the basic block of the if statement.

Have we got different opinions about the shown implementation details?

Regards,
Markus