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

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Date:   Wed, 19 Sep 2018 10:01:36 +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 19/09/18 08:15, Dongli Zhang wrote:
> Hi Juergen,
> 
> On 09/14/2018 10:44 PM, Juergen Gross wrote:
>> 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;
> 
> I recall what was happened.
> 
> Suppose one sample of path is "backend/vif/19/3/state". (we would like to obtain
> domid=19)
> 
> Initially I would like to use sscanf(path, "backend/%*[a-z]/%hu/%*u") to obtain
> the domid from xenstore path in one call.
> 
> Unfortunately, unlike userspace sscanf(), the version in linux kernel does not
> support '[' so that I would not be able to use "%*[a-z]" in sscanf() in linux
> kernel.

That is not correct. It doesn't support ranges in [], but it is
perfectly fine to use %[^/]. This requires a temporary buffer, as
%*[ isn't supported.

Why don't you use:

char temp[16];

...

/* kernel sscanf() %[] doesn't support '*' modifier and needs length. */
sscanf(path, "backend/%15[^/]%hu/%*u", temp, &domid)


Juergen