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

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Message-ID: <20171222154250.GB9630@fieldses.org>
Date:   Fri, 22 Dec 2017 10:42:50 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
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.

--b.