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

[Ananyev, Konstantin]

> -----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.
But ok, there is probably no point to argue on that one any longer -
let's keep the lock here, nothing will be broken with it for sure.

> 
> That said, I really have to wonder why ownership ids are really needed here at
> all.  It seems this design could be much simpler with the addition of a per-port
> lock (and optional ownership record).  The API could consist of three
> operations:
> 
> ownership_set
> ownership_tryset
> ownership_release
> ownership_get
> 

Ok, but how to distinguish who is the current owner of the port?
To make sure that only owner is allowed to perform control ops?
Konstantin

> 
> The first call simply tries to take the per-port lock (blocking if its already
> locked)
> 
> The second call is a non-blocking version of the first
> 
> The third unlocks the port, allowing others to take ownership
> 
> The fourth returns whatever ownership record you want to encode with the lock.
> 
> The addition of all this id checking seems a bit overcomplicated
> 
> Neil