Re: [dpdk-dev] [PATCH 2/2] net/failsafe: fix primary/secondary mutex

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 6/8/21 11:48 PM, Stephen Hemminger wrote:
> On Tue, 8 Jun 2021 18:55:17 +0300
> Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru> wrote:
> 
>> On 6/8/21 6:42 PM, Stephen Hemminger wrote:
>>> On Tue, 8 Jun 2021 11:00:37 +0300
>>> Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru> wrote:
>>>   
>>>> On 4/19/21 8:08 PM, Thomas Monjalon wrote:  
>>>>> About the title, better to speak about multi-process,
>>>>> it is less confusing than primary/secondary.
>>>>>
>>>>> 15/03/2021 20:27, Stephen Hemminger:    
>>>>>> Set mutex used in failsafe driver to protect when used by
>>>>>> both primary and secondary process. Without this fix, the failsafe
>>>>>> lock is not really locking when there are multiple secondary processes.
>>>>>>
>>>>>> Bugzilla ID: 662
>>>>>> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
>>>>>> Fixes: 655fcd68c7d2 ("net/failsafe: fix hotplug races")
>>>>>> Cc: matan@mellanox.com    
>>>>>
>>>>> The correct order for above lines is:
>>>>>
>>>>> Bugzilla ID: 662
>>>>> Fixes: 655fcd68c7d2 ("net/failsafe: fix hotplug races")
>>>>>
>>>>> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
>>>>>     
>>>>>> ---
>>>>>> --- a/drivers/net/failsafe/failsafe.c
>>>>>> +++ b/drivers/net/failsafe/failsafe.c
>>>>>> @@ -140,6 +140,11 @@ fs_mutex_init(struct fs_priv *priv)
>>>>>>  		ERROR("Cannot initiate mutex attributes - %s", strerror(ret));
>>>>>>  		return ret;
>>>>>>  	}
>>>>>> +	/* Allow mutex to protect primary/secondary */
>>>>>> +	ret = pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
>>>>>> +	if (ret)
>>>>>> +		ERROR("Cannot set mutex shared - %s", strerror(ret));    
>>>>>
>>>>> Why not returning an error here?    
>>>>
>>>> +1
>>>>
>>>> I think it would be safer to return an error here.  
>>>
>>> Ok but it never happens.
>>>   
>>
>> May I ask why? 'man pthread_mutexattr_setpshared' says that it
>> is possible.
>>
> 
> The glibc implementation of pthread_mutexattr_setpshared is:
> 
> 
> int
> pthread_mutexattr_setpshared (pthread_mutexattr_t *attr, int pshared)
> {
>   struct pthread_mutexattr *iattr;
> 
>   int err = futex_supports_pshared (pshared);
>   if (err != 0)
>     return err;
> 
>   iattr = (struct pthread_mutexattr *) attr;
> 
>   if (pshared == PTHREAD_PROCESS_PRIVATE)
>     iattr->mutexkind &= ~PTHREAD_MUTEXATTR_FLAG_PSHARED;
>   else
>     iattr->mutexkind |= PTHREAD_MUTEXATTR_FLAG_PSHARED;
> 
>   return 0;
> }
> 
> And
> 
> /* FUTEX_SHARED is always supported by the Linux kernel.  */
> static __always_inline int
> futex_supports_pshared (int pshared)
> {
>   if (__glibc_likely (pshared == PTHREAD_PROCESS_PRIVATE))
>     return 0;
>   else if (pshared == PTHREAD_PROCESS_SHARED)
>     return 0;
>   else
>     return EINVAL;
> }
> 
> 
> There for the code as written can not return an error.
> The check was only because someone could report a bogus
> issue from a broken c library.
> 

Many thanks for detailed description.
I thought that it is better to follow API
definition and it is not that hard to check
return code and handle it. Yes, glibc is not
the only C library.