[dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys

Van Haaren, Harry harry.van.haaren at intel.com
Tue Oct 2 15:17:00 CEST 2018


> -----Original Message-----
> From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli at arm.com]
> Sent: Sunday, September 30, 2018 11:33 PM
> To: Van Haaren, Harry <harry.van.haaren at intel.com>; Richardson, Bruce
> <bruce.richardson at intel.com>; Wang, Yipeng1 <yipeng1.wang at intel.com>
> Cc: De Lara Guarch, Pablo <pablo.de.lara.guarch at intel.com>; dev at dpdk.org;
> Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>; Steve Capper
> <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>; nd
> <nd at arm.com>
> Subject: RE: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving
> keys
> 
> 
> > > > >
> > > > >Reader-writer concurrency issue, caused by moving the keys to their
> > > > >alternative locations during key insert, is solved by introducing a
> > > > >global counter(tbl_chng_cnt) indicating a change in table.
> >
> > <snip>
> >
> > > > > /**
> > > > >@@ -200,7 +200,7 @@ rte_hash_add_key_with_hash_data(const struct
> > > > >rte_hash
> > > *h, const void *key,
> > > > >  *     array of user data. This value is unique for this key.
> > > > >  */
> > > > > int32_t
> > > > >-rte_hash_add_key(const struct rte_hash *h, const void *key);
> > > > >+rte_hash_add_key(struct rte_hash *h, const void *key);
> > > > >
> > > > > /**
> > > > >  * Add a key to an existing hash table.
> > > > >@@ -222,7 +222,7 @@ rte_hash_add_key(const struct rte_hash *h,
> > > > >const void
> > > *key);
> > > > >  *     array of user data. This value is unique for this key.
> > > > >  */
> > > > > int32_t
> > > > >-rte_hash_add_key_with_hash(const struct rte_hash *h, const void
> > > > >*key,
> > > hash_sig_t sig);
> > > > >+rte_hash_add_key_with_hash(struct rte_hash *h, const void *key,
> > > hash_sig_t sig);
> > > > >
> > > > > /
> > > >
> > > > I think the above changes will break ABI by changing the parameter
> type?
> > > Other people may know better on this.
> > >
> > > Just removing a const should not change the ABI, I believe, since the
> > > const is just advisory hint to the compiler. Actual parameter size and
> > > count remains unchanged so I don't believe there is an issue.
> > > [ABI experts, please correct me if I'm wrong on this]
> >
> >
> > [Certainly no ABI expert, but...]
> >
> > I think this is an API break, not ABI break.
> >
> > Given application code as follows, it will fail to compile - even though
> running
> > the new code as a .so wouldn't cause any issues (AFAIK).
> >
> > void do_hash_stuff(const struct rte_hash *h, ...) {
> >     /* parameter passed in is const, but updated function prototype is
> non-
> > const */
> >     rte_hash_add_key_with_hash(h, ...);
> > }
> >
> > This means that we can't recompile apps against latest patch without
> > application code changes, if the app was passing a const rte_hash struct
> as
> > the first parameter.
> >
> Agree. Do we need to do anything for this?

I think we should try to avoid breaking API wherever possible.
If we must, then I suppose we could follow the ABI process of
a deprecation notice.

>From my reading of the versioning docs, it doesn't document this case:
https://doc.dpdk.org/guides/contributing/versioning.html

I don't recall a similar situation in DPDK previously - so I suggest
you ask Tech board for input here.

Hope that helps! -Harry


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