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

[Neil Horman <nhorman@tuxdriver.com>]

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

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


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