should mutexes be uniquely named?

[Rick Macklem]

Kostik wrote:
> On Sat, Nov 28, 2015 at 04:40:44PM -0500, Rick Macklem wrote:
> > Kostik wrote:
> > > On Sat, Nov 28, 2015 at 08:29:55AM -0500, Rick Macklem wrote:
> > > > Hi,
> > > > 
> > > > I think the patches I posted last week that add "-manage-gids" are
> > > > about
> > > > ready for a commit to head.
> > > > 
> > > > However, there is one place in the code where I'm not sure which is
> > > > better
> > > > to do:
> > > > --> The code replaces a single mutex with one for each hash list head
> > > > (table
> > > >     entry).
> > > >     I currently use MTX_DUPOK and call them all the same thing.
> > > > or
> > > >     I could add a "lockname" field to the hash table enty structure and
> > > >     give
> > > >     each one a unique name (similar to what Garrett Wollman did in the
> > > >     kernel rpc).
> > > >     The only downside to this is 16bytes of storage for each hash table
> > > >     entry.
> > > >     (Admittedly, I don't think many sites would need to set the hash
> > > >     table
> > > >     size
> > > >      greater than a few thousand, so this isn't a lot of malloc()'d
> > > >      memory.)
> > > Question is, why do you need to acquire two mutexes simultaneously ?
> > > If mutexes protect the hash list rooted in head, then this is somewhat
> > > unusual.
> > > 
> > There are two hash tables, one hashed on names and the other uid/gid. The
> > entries are linked into both of these lists.
> > I suppose that I could use a different name for the "name" hash table
> > entries
> > vs the "uid/gid" ones, which would avoid the duplication for the common
> > cases.
> I think this is the easiest, together with ...
> 
> > 
> > There are also a couple of infrequent cases (when new entries are being
> > added
> > to the cache) where, to avoid a LOR in mutex locking the above 2 hash
> > tables,
> > the code locks all the table entries in the one hash table before doing the
> > other hash table. In this case, you will still end up with duplicates
> > unless
> > each lock is uniquely named.
> ... using mtx_lock_flags(MTX_DUPOK), to only shut up witness where it is
> neccessary.
> 
> > 
> > Maybe I should use a different name for the "user/group name" hash table
> > than
> > the "uid/gid" one, but still allow duplicates for the infrequent cases?
> Exactly.
> 
Thanks, that's what I will do unless others post with a differing opinion.

rick

> > 
> > Thanks for any help, rick
> > 
> > > Downside is not only the name, but also a witness overhead in the
> > > non-production kernels.
> > > 
> > > 
> > > > 
> > > > So, what do you think. Should I add the code to make the mutex names
> > > > unique?
> > > > 
> > > > Thanks in advance for any comments, rick
> > > > ps: The coding change is trivial. It just involves using more
> > > > malloc()'d
> > > > memory.
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>