linux-kernel - RE: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to refcount

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Date:   Wed, 27 Dec 2017 12:10:15 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
CC:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "trond.myklebust@...marydata.com" <trond.myklebust@...marydata.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to
 refcount_t

> 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?
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.


Best Regards,
Elena
> 
> --b.