On Wed, Dec 27, 2017 at 12:10:15PM +0000, Reshetova, Elena wrote:
> > On Fri, Dec 22, 2017 at 09:25:53AM -0500, J. Bruce Fields wrote:
> > > On Fri, Dec 22, 2017 at 09:29:15AM +0000, Reshetova, Elena wrote:
> > > >
> > > > On Wed, Nov 29, 2017 at 01:15:43PM +0200, Elena Reshetova wrote:
> > > > > atomic_t variables are currently used to implement reference
> > > > > counters with the following properties:
> > > > > - counter is initialized to 1 using atomic_set()
> > > > > - a resource is freed upon counter reaching zero
> > > > > - once counter reaches zero, its further
> > > > > increments aren't allowed
> > > > > - counter schema uses basic atomic operations
> > > > > (set, inc, inc_not_zero, dec_and_test, etc.)
> > > >
> > > > >Whoops, I forgot that this doesn't apply to h_count.
> > > >
> > > > >Well, it's confusing, because h_count is actually used in two different
> > > > >ways: depending on whether a nlm_host represents a client or server, it
> > > > >may have the above properties or not.
> > > >
> > > >
> > > > So, what happens when it is not having the above properties? Is the object
> > > > being reused or?
> > >
> > > The object isn't destroyed when the counter hits zero--zero is just
> > > taken as a hint to some garbage collection algorithm that it would be OK
> > > to destroy it. So decrementing to or incrementing from zero is OK.
> >
> > In more detail: the nlm_host objects that are used on the NFS server to
> > represent NFS clients are put by nlmsvc_release_host, and then may
> > eventually be freed by nlm_gc_hosts.
> >
> > The nlm_host objects that are used on the NFS client to represent NFS
> > servers are put (and freed when h_count goes to zero) by
> > nlmclnt_release_host.
> >
> > In both cases reference are taken by nlm_get_host. It would be possible
> > to replace nlm_get_host by two different functions if that would help.
> > Most callers are obviously only client-side or server-side. The only
> > exception is next_host_state. It could be passed a pointer to the "get"
> > function it should use.
> >
> > After that we might actually just want to define separate client and
> > server structs like:
> >
> > struct nlm_clnt_host {
> > struct nlm_host ch_host;
> > refcount_t ch_count;
> > ...
> > }
> >
> > struct nlm_srv_host {
> > struct nlm_host sh_host;
> > refcount_t sh_count;
> > ...
> > }
> >
> > rather than have a single h_count which is used in two confusingly
> > different ways. There are also some other nlm_host fields that really
> > only make sense for client or server.
>
> This sounds reasonable for me, but obviously it is a bigger change and I might not
> have enough knowledge on NFS to make it correctly.
>
> In any case, even for the current server case, when freeing might not happen and object gets
> re-used later on, is it possible to simply re-initialize the object (and its reference counter) properly before reusing?
The object still has useful information in it so we can't just
reinitalize it completely. I guess we could make nlm_get_host do
if (refcount_read(&host->h_count))
refcount_inc(&host->h_count);
else
refcount_set(&host->h_count, 1);
Or we could just change the code so the refcount is always 1 higher in
the NFS server case, so "1" instead of "0" is used to mean "nobody's
using this, you can garbage collect this", and then it won't go to 0
until the garbage collector actually destroys it.
This isn't an unusual pattern, what have other subsystems been doing?
--b.
> I think this is the only thing that is needed from the correct refcounting POV in this case, so
> instead of using refcount_inc() on reused object, you would explicitly do refcount_set(counter, 1) when reuse happens.
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