[dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists
konstantin.ananyev at intel.com
Wed Jun 15 11:54:09 CEST 2016
> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
> Sent: Wednesday, June 15, 2016 9:49 AM
> To: Ananyev, Konstantin
> Cc: Pattan, Reshma; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists
> 2016-06-15 08:37, Ananyev, Konstantin:
> > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
> > > 2016-06-15 05:30, Pattan, Reshma:
> > > > From: Thomas Monjalon [mailto:thomas.monjalon at 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 at 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)
> > > >
> > > > > 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
> > > > 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.
Current status (16.04):
It is safe to add/remove RX/TX callbacks while
another thread is doing simultaneously RX/TX burst over same queue.
I.E: it is supposed to be safe to invoke
rte_eth_add(/remove)_rx(/tx)_callback() and rte_eth_rx_burst()/rte_eth_tx_burst()
from different threads simultaneously.
Though it is not safe to free/modify that rte_eth_rxtx_callback while current
rte_eth_rx_burst()/rte_eth_tx_burst() are still active.
That exactly what comments for rte_eth_remove_rx_callback() say:
* Note: the callback is removed from the callback list but it isn't freed
* since the it may still be in use. The memory for the callback can be
* subsequently freed back by the application by calling rte_free():
* - Immediately - if the port is stopped, or the user knows that no
* callbacks are in flight e.g. if called from the thread doing RX/TX
* on that queue.
* - After a short delay - where the delay is sufficient to allow any
* in-flight callbacks to complete.
In other words, right now there only way to know for sure that it is safe
to free the removed callback - is to stop the port.
Does it need to be changed, so when rte_eth_remove_rx_callback() returns
user can safely free the callback (or even better rte_eth_remove_rx_callback free the callback for us)?
In my opinion - yes.
Though, I think, it has nothing to do with pdump patches, and I think should be a matter
for separate a patch/discussion.
Now with pdump library introduction - there is possibility that 2 different threads
can try to add/remove callbacks for the same queue simultaneously.
First one - thread executing control requests from local user,
second one - pdump control thread executing pdump requests from pdump client.
That lock is introduced to avoid race condition between such 2 threads:
i.e. to prevent multiple threads to modify same list simultaneously.
It is not intended to synchronise read/write accesses to the list, see above.
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