linux-kernel - Re: [Xen-devel] [PATCH 5/6] xenbus: process be

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Date:   Fri, 14 Sep 2018 16:44:09 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     wei.liu2@...rix.com, konrad.wilk@...cle.com,
        srinivas.eeda@...cle.com, paul.durrant@...rix.com,
        boris.ostrovsky@...cle.com, roger.pau@...rix.com
Subject: Re: [Xen-devel] [PATCH 5/6] xenbus: process be_watch events in
 xenwatch multithreading

On 14/09/18 16:29, Dongli Zhang wrote:
> Hi Juergen,
> 
> On 09/14/2018 10:26 PM, Juergen Gross wrote:
>> On 14/09/18 16:18, Dongli Zhang wrote:
>>> Hi Juergen,
>>>
>>> On 09/14/2018 05:12 PM, Juergen Gross wrote:
>>>> On 14/09/18 09:34, Dongli Zhang wrote:
>>>>> This is the 5th patch of a (6-patch) patch set.
>>>>>
>>>>> With this patch, watch event in relative path pattern
>>>>> 'backend/<pvdev>/<domid>i/...' can be processed in per-domU xenwatch
>>>>
>>>> superfluous "i" ----------^
>>>>
>>>>> thread.
>>>>>
>>>>> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
>>>>> ---
>>>>>  drivers/xen/xenbus/xenbus_probe.c         |  2 +-
>>>>>  drivers/xen/xenbus/xenbus_probe_backend.c | 32 +++++++++++++++++++++++++++++++
>>>>>  include/xen/xenbus.h                      |  2 ++
>>>>>  3 files changed, 35 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
>>>>> index ba0644c..aa1b15a 100644
>>>>> --- a/drivers/xen/xenbus/xenbus_probe.c
>>>>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>>>>> @@ -552,7 +552,7 @@ int xenbus_probe_devices(struct xen_bus_type *bus)
>>>>>  }
>>>>>  EXPORT_SYMBOL_GPL(xenbus_probe_devices);
>>>>>  
>>>>> -static unsigned int char_count(const char *str, char c)
>>>>> +unsigned int char_count(const char *str, char c)
>>>>
>>>> Please change the name of the function when making it globally
>>>> visible, e.g. by prefixing "xenbus_".
>>>>
>>>> Generally I think you don't need to use it below.
>>>>
>>>>>  {
>>>>>  	unsigned int i, ret = 0;
>>>>>  
>>>>> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
>>>>> index b0bed4f..50df86a 100644
>>>>> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
>>>>> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
>>>>> @@ -211,9 +211,41 @@ static void backend_changed(struct xenbus_watch *watch,
>>>>>  	xenbus_dev_changed(path, &xenbus_backend);
>>>>>  }
>>>>>  
>>>>> +static domid_t path_to_domid(const char *path)
>>>>> +{
>>>>> +	const char *p = path;
>>>>> +	domid_t domid = 0;
>>>>> +
>>>>> +	while (*p) {
>>>>> +		if (*p < '0' || *p > '9')
>>>>> +			break;
>>>>> +		domid = (domid << 3) + (domid << 1) + (*p - '0');
>>>>
>>>> reinventing atoi()?
>>>>
>>>> Please don't do that. kstrtou16() seems to be a perfect fit.
>>>
>>> I did use kstrtou*() in the early prototype and realized kstrtou16() returns 0
>>> if the input string contains non-numerical characters.
>>>
>>> E.g., the example of input can be "1/0/state", where 1 is fotherend_id
>>> (frontend_id) and 0 is handle.
>>>
>>> When "1/0/state" is used at input, kstrtou16() returns 0 (returned integer) and
>>> -22 (error).
>>
>> Aah, okay. Then simple_strtoul()?
> 
> I did consider simple_strtoul() initially. Unfortunately, it is obsolete (below
> line 81). AFAIR, the patch would not be able to pass the check_patch script when
> this function is used.

Better use that than open coding a new instance of it.

Another variant would be to use sscanf() or similar. Then you could even
drop using strchr() by adding that in the format string:

return (sscanf(path, "%*u/%u/", &domid) == 1) ? domid : DOMID_SELF;


Juergen