Hi Neil From: Neil Horman, Thursday, January 18, 2018 6:55 PM > 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:nhor...@tuxdriver.com] > > > > > Sent: Wednesday, January 17, 2018 2:00 PM > > > > > To: Matan Azrad <ma...@mellanox.com> > > > > > Cc: Ananyev, Konstantin <konstantin.anan...@intel.com>; Thomas > > > > > Monjalon <tho...@monjalon.net>; Gaetan Rivet > > > > > <gaetan.ri...@6wind.com>; Wu, Jingjing <jingjing...@intel.com>; > > > > > dev@dpdk.org; Richardson, Bruce <bruce.richard...@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.
Why? Each one will get its owner id according to some order(The critical section is protected by spinlock). > 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). > Yes. > 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. Yes. > 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. > Yes, I understand what you say - there is no deterministic state for ownership set success. Actually I think it will be very hard to arrive to determination in DPDK regarding port ownership when multi-thread is in the game, Especially it depend in a lot of DPDK entities implementation.. But the current non-deterministic approach makes good order in the game. > Regards > Neil > > > > Neil > > > >
