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

[Stephen Hemminger <stephen@networkplumber.org>]

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.