[dpdk-dev] [PATCH v2 2/6] ethdev: add port ownership

[Neil Horman]

On Thu, Jan 18, 2018 at 02:00:23PM +0000, Matan Azrad wrote:
> Hi Neil
> 
> From: Neil Horman, Thursday, January 18, 2018 3:10 PM
> > On Wed, Jan 17, 2018 at 05:01:10PM +0000, Ananyev, Konstantin wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Neil Horman [mailto:nhorman at tuxdriver.com]
> > > > Sent: Wednesday, January 17, 2018 2:00 PM
> > > > To: Matan Azrad <matan at mellanox.com>
> > > > Cc: Ananyev, Konstantin <konstantin.ananyev at intel.com>; Thomas
> > > > Monjalon <thomas at monjalon.net>; Gaetan Rivet
> > > > <gaetan.rivet at 6wind.com>; Wu, Jingjing <jingjing.wu at intel.com>;
> > > > dev at dpdk.org; Richardson, Bruce <bruce.richardson at intel.com>
> > > > Subject: Re: [PATCH v2 2/6] ethdev: add port ownership
> > > >
> > > > On Wed, Jan 17, 2018 at 12:05:42PM +0000, Matan Azrad wrote:
> > > > >
> > > > > Hi Konstantin
> > > > > From: Ananyev, Konstantin, Sent: Wednesday, January 17, 2018 1:24
> > > > > PM
> > > > > > Hi Matan,
> > > > > >
> > > > > > > Hi Konstantin
> > > > > > >
> > > > > > > From: Ananyev, Konstantin, Tuesday, January 16, 2018 9:11 PM
> > > > > > > > Hi Matan,
> > > > > > > >
> > > > > > > > >
> > > > > > > > > Hi Konstantin
> > > > > > > > > From: Ananyev, Konstantin, Monday, January 15, 2018 8:44
> > > > > > > > > PM
> > > > > > > > > > Hi Matan,
> > > > > > > > > > > Hi Konstantin
> > > > > > > > > > > From: Ananyev, Konstantin, Monday, January 15, 2018
> > > > > > > > > > > 1:45 PM
> > > > > > > > > > > > Hi Matan,
> > > > > > > > > > > > > Hi Konstantin
> > > > > > > > > > > > > From: Ananyev, Konstantin, Friday, January 12,
> > > > > > > > > > > > > 2018 2:02 AM
> > > > > > > > > > > > > > Hi Matan,
> > > > > > > > > > > > > > > Hi Konstantin
> > > > > > > > > > > > > > > From: Ananyev, Konstantin, Thursday, January
> > > > > > > > > > > > > > > 11, 2018
> > > > > > > > > > > > > > > 2:40 PM
> > > > > > > > > > > > > > > > Hi Matan,
> > > > > > > > > > > > > > > > > Hi Konstantin
> > > > > > > > > > > > > > > > > From: Ananyev, Konstantin, Wednesday,
> > > > > > > > > > > > > > > > > January 10,
> > > > > > > > > > > > > > > > > 2018
> > > > > > > > > > > > > > > > > 3:36 PM
> > > > > > > > > > > > > > > > > > Hi Matan,
> > > > > > > > >  <snip>
> > > > > > > > > > > > > > > > > > It is good to see that now
> > > > > > > > > > > > > > > > > > scanning/updating rte_eth_dev_data[] is
> > > > > > > > > > > > > > > > > > lock protected, but it might be not very
> > > > > > > > > > > > > > > > > > plausible to protect both data[] and
> > > > > > > > > > > > > > > > > > next_owner_id using the
> > > > > > > > > > > > same lock.
> > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > I guess you mean to the owner structure in
> > > > > > > > > > > > rte_eth_dev_data[port_id].
> > > > > > > > > > > > > > > > > The next_owner_id is read by ownership
> > > > > > > > > > > > > > > > > APIs(for owner validation), so it
> > > > > > > > > > > > > > > > makes sense to use the same lock.
> > > > > > > > > > > > > > > > > Actually, why not?
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > Well to me next_owner_id and
> > > > > > > > > > > > > > > > rte_eth_dev_data[] are not directly
> > > > > > > > > > > > > > related.
> > > > > > > > > > > > > > > > You may create new owner_id but it doesn't
> > > > > > > > > > > > > > > > mean you would update rte_eth_dev_data[]
> > immediately.
> > > > > > > > > > > > > > > > And visa-versa - you might just want to
> > > > > > > > > > > > > > > > update rte_eth_dev_data[].name or .owner_id.
> > > > > > > > > > > > > > > > It is not very good coding practice to use
> > > > > > > > > > > > > > > > same lock for non-related data structures.
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > I see the relation like next:
> > > > > > > > > > > > > > > Since the ownership mechanism synchronization
> > > > > > > > > > > > > > > is in ethdev responsibility, we must protect
> > > > > > > > > > > > > > > against user mistakes as much as we can by
> > > > > > > > > > > > > > using the same lock.
> > > > > > > > > > > > > > > So, if user try to set by invalid owner
> > > > > > > > > > > > > > > (exactly the ID which currently is
> > > > > > > > > > > > > > allocated) we can protect on it.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Hmm, not sure why you can't do same checking
> > > > > > > > > > > > > > with different lock or atomic variable?
> > > > > > > > > > > > > >
> > > > > > > > > > > > > The set ownership API is protected by ownership
> > > > > > > > > > > > > lock and checks the owner ID validity By reading the next
> > owner ID.
> > > > > > > > > > > > > So, the owner ID allocation and set API should use
> > > > > > > > > > > > > the same atomic
> > > > > > > > > > > > mechanism.
> > > > > > > > > > > >
> > > > > > > > > > > > Sure but all you are doing for checking validity, is
> > > > > > > > > > > > check that owner_id > 0 &&& owner_id < next_ownwe_id,
> > right?
> > > > > > > > > > > > As you don't allow owner_id overlap (16/3248 bits)
> > > > > > > > > > > > you can safely do same check with just
> > atomic_get(&next_owner_id).
> > > > > > > > > > > >
> > > > > > > > > > > It will not protect it, scenario:
> > > > > > > > > > > - current next_id is X.
> > > > > > > > > > > - call set ownership of port A with owner id X by
> > > > > > > > > > > thread 0(by user
> > > > > > > > mistake).
> > > > > > > > > > > - context switch
> > > > > > > > > > > - allocate new id by thread 1 and get X and change
> > > > > > > > > > > next_id to
> > > > > > > > > > > X+1
> > > > > > > > > > atomically.
> > > > > > > > > > > -  context switch
> > > > > > > > > > > - Thread 0 validate X by atomic_read and succeed to
> > > > > > > > > > > take
> > > > > > ownership.
> > > > > > > > > > > - The system loosed the port(or will be managed by two
> > > > > > > > > > > entities) -
> > > > > > > > crash.
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Ok, and how using lock will protect you with such scenario?
> > > > > > > > >
> > > > > > > > > The owner set API validation by thread 0 should fail
> > > > > > > > > because the owner
> > > > > > > > validation is included in the protected section.
> > > > > > > >
> > > > > > > > Then your validation function would fail even if you'll use
> > > > > > > > atomic ops instead of lock.
> > > > > > > No.
> > > > > > > With atomic this specific scenario will cause the validation to pass.
> > > > > >
> > > > > > Can you explain to me how?
> > > > > >
> > > > > > rte_eth_is_valid_owner_id(uint16_t owner_id) {
> > > > > >               int32_t cur_owner_id =
> > > > > > RTE_MIN(rte_atomic32_get(next_owner_id),
> > > > > > UINT16_MAX);
> > > > > >
> > > > > > 	if (owner_id == RTE_ETH_DEV_NO_OWNER || owner >
> > > > > > cur_owner_id) {
> > > > > > 		RTE_LOG(ERR, EAL, "Invalid owner_id=%d.\n", owner_id);
> > > > > > 		return 0;
> > > > > > 	}
> > > > > > 	return 1;
> > > > > > }
> > > > > >
> > > > > > Let say your next_owne_id==X, and you invoke
> > > > > > rte_eth_is_valid_owner_id(owner_id=X+1)  - it would fail.
> > > > >
> > > > > Explanation:
> > > > > The scenario with locks:
> > > > > next_owner_id = X.
> > > > > Thread 0 call to set API(with invalid owner Y=X) and take lock.
> > > > > Context switch.
> > > > > Thread 1 call to owner_new and stuck in the lock.
> > > > > Context switch.
> > > > > Thread 0 does owner id validation and failed(Y>=X) - unlock the lock and
> > return failure to the user.
> > > > > Context switch.
> > > > > Thread 1 take the lock and update X to X+1, then, unlock the lock.
> > > > > Everything is OK!
> > > > >
> > > > > The same scenario with atomics:
> > > > > next_owner_id = X.
> > > > > Thread 0 call to set API(with invalid owner Y=X) and take lock.
> > > > > Context switch.
> > > > > Thread 1 call to owner_new and change X to X+1(atomically).
> > > > > Context switch.
> > > > > Thread 0 does owner id validation and success(Y<(atomic)X+1) - unlock
> > the lock and return success to the  user.
> > > > > Problem!
> > > > >
> > > >
> > > >
> > > > Matan is correct here, there is no way to preform parallel set
> > > > operations using just and atomic variable here, because multiple
> > > > reads of next_owner_id need to be preformed while it is stable.
> > > > That is to say rte_eth_next_owner_id must be compared to
> > > > RTE_ETH_DEV_NO_OWNER and owner_id in rte_eth_is_valid_owner_id.
> > If
> > > > you were to only use an atomic_read on such a variable, it could be
> > > > incremented by the owner_new function between the checks and an
> > > > invalid owner value could become valid because  a third thread
> > > > incremented the next value.  The state of next_owner_id must be kept
> > > > stable during any validity checks
> > >
> > > It could still be incremented between the checks - if let say
> > > different thread will invoke new_onwer_id, grab the lock update
> > > counter, release the lock - all that before the check.
> > I don't see how all of the contents of rte_eth_dev_owner_set is protected
> > under rte_eth_dev_ownership_lock, as is rte_eth_dev_owner_new.
> > Next_owner might increment between another threads calls to owner_new
> > and owner_set, but that will just cause a transition from an ownership id
> > being valid to invalid, and thats ok, as long as there is consistency in the
> > model that enforces a single valid owner at a time (in that case the
> > subsequent caller to owner_new).
> > 
> 
> I'm not sure I fully understand you, but see:
> we can't protect all of the user mistakes(using the wrong owner id).
> But we are doing the maximum for it.
> 
Yeah, my writing was atrocious, apologies.  All I meant to say was that the
locking you have is ok, in that it maintains a steady state for the data being
read during the period its being read.  The fact that a given set operation may
fail because someone else created an ownership record is an artifact of the api,
not a bug in its implementation.  I think we're basically in agreement on the
semantics here, but this goes to my argument about complexity (more below).

> 
> > Though this confusion does underscore my assertion I think that this API is
> > overly complicated
> > 
> 
> I really don't think it is complicated. - just take ownership of a port(by owner id allocation and set APIs) and manage the port as you want. 
> 
But thats not all.  The determination of success or failure in claiming
ownership is largely dependent on the behavior of other threads actions, not a
function of the state of the system at the moment ownership is requested.  That
is to say, if you have N threads, and they all create ownership objects
identified as X, x+1, X+2...X+N, only the thread with id X+N will be able to
claim ownership of any port, because they all will have incremented the shared
nex_id variable.  Determination of ownership by the programmer will have to be
done via debugging, and errors will likely be transient dependent on the order
in which threads execute (subject to scheduling jitter).  

Rather than making ownership success dependent on any data contained within the
ownership record, ownership should be entirely dependent on the state of port
ownership at the time that it was requested.  That is to say, port ownership
should succede if and only if the port is unowned at the time that a given
thread requets ownership.  Any ancilliary data regarding which context owns the
port should be exactly that, ancilliary, and have no impact on weather or not
the port ownership request succedes.

Regards
Neil

> > Neil
> 
>