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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-06-15 08:37, Ananyev, Konstantin:
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > 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.  

If we add/remove a callback during a burst? Is it possible that the next
pointer would have a wrong value leading to a crash?
Maybe we need a comment to state that we should not alter burst
callbacks while running burst functions.