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

Ananyev, Konstantin konstantin.ananyev at intel.com
Mon Jan 15 12:45:03 CET 2018


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>
> > > > > > Few comments from me below.
> > > > > > BTW, do you plan to add ownership mandatory check in control
> > > > > > path functions that change port configuration?
> > > > >
> > > > > No.
> > > >
> > > > So it still totally voluntary usage and application nneds to be
> > > > changed to exploit it?
> > > > Apart from RTE_FOR_EACH_DEV() change proposed by Gaetan?
> > > >
> > >
> > > Also RTE_FOR_EACH_DEV() change proposed by Gaetan is not protected
> > because 2 DPDK entities can get the same port while using it.
> >
> > I am not talking about racing condition here.
> > Right now even from the same thread - I can call dev_configure() for the port
> > which I don't own (let say it belongs to failsafe port), and that would remain,
> > correct?
> >
> Yes.

Ok, thanks for clarification.
I think that makes current approach sort of incomplete, but might be it is a 
subject of separate discussion.

> 
> > > As I wrote in the log\docs and as discussed a lot in the first version:
> > > The new synchronization rules are:
> > > 1. The port allocation and port release synchronization will be
> > >    managed by ethdev.
> > > 2. The port usage synchronization will be managed by the port owner.
> > > 3. The port ownership API synchronization(also with port creation) will be
> > managed by ethdev.
> > > 4. DPDK entity which want to use a port must take ownership before.
> > >
> > > Ethdev should not protect 2 and 4 according these rules.
> > >
> > > > > > Konstantin
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: Matan Azrad [mailto:matan at mellanox.com]
> <snip>
> > > I mean the documentation about the needed alignment for spinlock.
> > Where is it?
> >
> > https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Finfo
> > center.arm.com%2Fhelp%2Findex.jsp%3Ftopic%3D%2Fcom.arm.doc.faqs%2
> > Fka15414.html&data=02%7C01%7Cmatan%40mellanox.com%7Cb3c329ae9db
> > f4bd29a7008d5594fb776%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C1
> > %7C636513121294703050&sdata=40v3b4wk5f4qEyIY5jdDv8S47LjgXK0t9TPtav
> > XIMOk%3D&reserved=0
> > https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Finfo
> > center.arm.com%2Fhelp%2Findex.jsp%3Ftopic%3D%2Fcom.arm.doc.dht000
> > 8a%2FCJAGCFAF.html&data=02%7C01%7Cmatan%40mellanox.com%7Cb3c32
> > 9ae9dbf4bd29a7008d5594fb776%7Ca652971c7d2e4d9ba6a4d149256f461b%7
> > C0%7C1%7C636513121294703050&sdata=B7pEZjFJntVp3Il8fS9wr%2FlxABgNX
> > FSr9PE4emEPLQE%3D&reserved=0
> >
> > Might be ARM and PPC guys can provide you some more complete/recent
> > docs.
> Thanks.
> <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). 

> The set(and others) ownership APIs already uses the ownership lock so I think it makes sense to use the same lock also in ID allocation.
> 
> > > > > > In fact, for next_owner_id, you don't need a lock - just
> > > > > > rte_atomic_t should be enough.
> > > > >
> > > > > I don't think so, it is problematic in next_owner_id wraparound
> > > > > and may
> > > > complicate the code in other places which read it.
> > > >
> > > > IMO it is not that complicated, something like that should work I think.
> > > >
> > > > /* init to 0 at startup*/
> > > > rte_atomic32_t *owner_id;
> > > >
> > > > int new_owner_id(void)
> > > > {
> > > >     int32_t x;
> > > >     x = rte_atomic32_add_return(&owner_id, 1);
> > > >     if (x > UINT16_MAX) {
> > > >        rte_atomic32_dec(&owner_id);
> > > >        return -EOVERWLOW;
> > > >     } else
> > > >         return x;
> > > > }
> > > >
> > > >
> > > > > Why not just to keep it simple and using the same lock?
> > > >
> > > > Lock is also fine, I just think it better be a separate one - that
> > > > would protext just next_owner_id.
> > > > Though if you are going to use uuid here - all that probably not
> > > > relevant any more.
> > > >
> > >
> > > I agree about the uuid but still think the same lock should be used for both.
> >
> > But with uuid you don't need next_owner_id at all, right?
> > So lock will only be used for rte_eth_dev_data[] fields anyway.
> >
> Sorry, I meant uint64_t, not uuid.

Ah ok, my thought uuid_t is better as with it you don't need to support your own code
to allocate new owner_id, but rely on system libs instead.
But wouldn't insist here.

> 
> > > > > > Another alternative would be to use 2 locks - one for
> > > > > > next_owner_id second for actual data[] protection.
> > > > > >
> > > > > > Another thing - you'll probably need to grab/release a lock
> > > > > > inside
> > > > > > rte_eth_dev_allocated() too.
> > > > > > It is a public function used by drivers, so need to be protected too.
> > > > > >
> > > > >
> > > > > Yes, I thought about it, but decided not to use lock in next:
> > > > > rte_eth_dev_allocated
> > > > > rte_eth_dev_count
> > > > > rte_eth_dev_get_name_by_port
> > > > > rte_eth_dev_get_port_by_name
> > > > > maybe more...
> > > >
> > > > As I can see in patch #3 you protect by lock access to
> > > > rte_eth_dev_data[].name (which seems like a good  thing).
> > > > So I think any other public function that access
> > > > rte_eth_dev_data[].name should be protected by the same lock.
> > > >
> > >
> > > I don't think so, I can understand to use the ownership lock here(as in port
> > creation) but I don't think it is necessary too.
> > > What are we exactly protecting here?
> > > Don't you think it is just timing?(ask in the next moment and you  may
> > > get another answer) I don't see optional crash.
> >
> > Not sure what you mean here by timing...
> > As I understand rte_eth_dev_data[].name unique identifies device and is
> > used by  port allocation/release/find functions.
> > As you stated above:
> > "1. The port allocation and port release synchronization will be  managed by
> > ethdev."
> > To me it means that ethdev layer has to make sure that all accesses to
> > rte_eth_dev_data[].name are atomic.
> > Otherwise what would prevent the situation when one process does
> > rte_eth_dev_allocate()->snprintf(rte_eth_dev_data[x].name, ...) while
> > second one does rte_eth_dev_allocated(rte_eth_dev_data[x].name, ...) ?
> >
> The second will get True or False and that is it.

Under race condition - in the worst case it might crash, though for that you'll have to be really unlucky.
Though in most cases as you said it would just not operate correctly.
I think if we start to protect dev->name by lock we need to do it for all instances
(both read and write).  

> Maybe if it had been called just a moment after, It might get different answer.
> Because these APIs don't change ethdev structure(just read), it can be OK.
> But again, I can understand to use ownership lock also here.
> 

Konstantin


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