Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Thomas,

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Wednesday, June 15, 2016 9:19 AM
> To: Pattan, Reshma
> Cc: dev@dpdk.org; Ananyev, Konstantin
> Subject: Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists
> 
> 2016-06-15 05:30, Pattan, Reshma:
> > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > > 2016-06-14 10:38, Reshma Pattan:
> > > > Added spinlocks around add/remove logic of Rx and Tx callbacks to
> > > > avoid corruption of callback lists in multithreaded context.
> > > >
> > > > Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
> > >
> > > Why cb->next is not locked in burst functions?
> > It is safe to do "read access" here and doesn't require any locking as rx/tx burst is initiated  by only local user(control plane) thread.
> >
> > > Just protecting add/remove but not its usage seems useless.
> > Here locks were required  around add/remove to protect "write access"  because write to callback list is now done from 2 threads
> > i.e. one from local user thread(control plane) and another from pdump control thread(initiated by remote pdump request).
> 
> So read and write can be done by different threads.

Yes, and this is possible even in current DPDK version (16.04).
What is added by Reshma's patch - now it is possible to have concurrent write
from 2 different thread to that list.  

> I think the read access would need locking but we do not want it
> in fast path.

I don't think it would be needed.
As I said - read/write interaction didn't change from what we have right now.
But if you have some particular scenario in mind that you believe would cause
a race condition - please speak up.  
Konstantin

> Are you sure there is no issue in this design?