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

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Message-ID: <a85d7634-b62b-7ac9-8ce9-c548b02afb1e@oracle.com>
Date:   Wed, 19 Sep 2018 20:27:26 +0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     Juergen Gross <jgross@...e.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

Hi Juergen,

On 09/19/2018 04:01 PM, Juergen Gross wrote:
> 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)
> 

One '\' between "%15[^/]" and "%hu" is missing.

We should use "backend/%15[^/]/%hu/%*u" instead.

Seems this is supported since commit f9310b2f9a19b7f16c7b1c1558f8b649b9b933c1.
Only tag since 4.6 support this feature.

I should avoid using old ubuntu 4.4.0 kernel to test such features the next time :(

Thank you very much for your help!

As the "devicetype[32]" in struct xenbus_device_id is of size 32, should I use
temp[32] instead of temp[16]?

Dongli Zhang