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

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Message-ID: <1516754851.18068.3.camel@gmail.com>
Date:   Tue, 23 Jan 2018 19:47:31 -0500
From:   Trond Myklebust <trondmy@...il.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>,
        "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>,
        "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 Tue, 2018-01-23 at 17:09 -0500, J. Bruce Fields wrote:
> 
> 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?
> 

Hi Bruce,

Sorry I forgot about the issues with the server garbage collector, and
I applied these patches to my linux-next a couple of weeks ago.

What say we fix the issue with something like the following?

8<------------------------------------------------------------
>From 83ce0f55ca54337a573f1d70038714815a9cd645 Mon Sep 17 00:00:00 2001
From: Trond Myklebust <trondmy@...il.com>
Date: Tue, 23 Jan 2018 19:39:04 -0500
Subject: [PATCH] lockd: Fix server refcounting

The server shouldn't actually delete the struct nlm_host until it hits
the garbage collector. In order to make that work correctly with the
refcount API, we can bump the refcount by one, and then use
refcount_dec_if_one() in the garbage collector.

Signed-off-by: Trond Myklebust <trondmy@...il.com>
---
 fs/lockd/host.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/lockd/host.c b/fs/lockd/host.c
index 7d6ab72bbe65..d35cd6be0675 100644
--- a/fs/lockd/host.c
+++ b/fs/lockd/host.c
@@ -388,6 +388,8 @@ struct nlm_host *nlmsvc_lookup_host(const struct svc_rqst *rqstp,
 	ln->nrhosts++;
 	nrhosts++;
 
+	refcount_inc(&host->h_count);
+
 	dprintk("lockd: %s created host %s (%s)\n",
 		__func__, host->h_name, host->h_addrbuf);
 
@@ -662,8 +664,7 @@ nlm_gc_hosts(struct net *net)
 	for_each_host_safe(host, next, chain, nlm_server_hosts) {
 		if (net && host->net != net)
 			continue;
-		if (refcount_read(&host->h_count) || host->h_inuse
-		 || time_before(jiffies, host->h_expires)) {
+		if (host->h_inuse || time_before(jiffies, host->h_expires)) {
 			dprintk("nlm_gc_hosts skipping %s "
 				"(cnt %d use %d exp %ld net %x)\n",
 				host->h_name, refcount_read(&host->h_count),
@@ -671,7 +672,8 @@ nlm_gc_hosts(struct net *net)
 				host->net->ns.inum);
 			continue;
 		}
-		nlm_destroy_host_locked(host);
+		if (refcount_dec_if_one(&host->h_count))
+			nlm_destroy_host_locked(host);
 	}
 
 	if (net) {
-- 
2.14.3

-- 
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@...marydata.com