nfsd: make nfsd_stats.th_cnt atomic_t

This allows us to move the updates for th_cnt out of the mutex.
This is a step towards reducing mutex coverage in nfsd().
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -57,7 +57,7 @@ static __be32			nfsd_init_request(struct svc_rqst *,
 /*
  * nfsd_mutex protects nn->nfsd_serv -- both the pointer itself and the members
  * of the svc_serv struct. In particular, ->sv_nrthreads but also to some
- * extent ->sv_temp_socks and ->sv_permsocks. It also protects nfsdstats.th_cnt
+ * extent ->sv_temp_socks and ->sv_permsocks.
  *
  * If (out side the lock) nn->nfsd_serv is non-NULL, then it must point to a
  * properly initialised 'struct svc_serv' with ->sv_nrthreads > 0 (unless
@@ -955,8 +955,8 @@ nfsd(void *vrqstp)
 	allow_signal(SIGINT);
 	allow_signal(SIGQUIT);
 
-	nfsdstats.th_cnt++;
 	mutex_unlock(&nfsd_mutex);
+	atomic_inc(&nfsdstats.th_cnt);
 
 	set_freezable();
 
@@ -983,8 +983,8 @@ nfsd(void *vrqstp)
 	/* Clear signals before calling svc_exit_thread() */
 	flush_signals(current);
 
+	atomic_dec(&nfsdstats.th_cnt);
 	mutex_lock(&nfsd_mutex);
-	nfsdstats.th_cnt --;
 
 out:
 	/* Take an extra ref so that the svc_put in svc_exit_thread()

fs/nfsd/stats.c

@@ -45,7 +45,7 @@ static int nfsd_proc_show(struct seq_file *seq, void *v)
 		   percpu_counter_sum_positive(&nfsdstats.counter[NFSD_STATS_IO_WRITE]));
 
 	/* thread usage: */
-	seq_printf(seq, "th %u 0", nfsdstats.th_cnt);
+	seq_printf(seq, "th %u 0", atomic_read(&nfsdstats.th_cnt));
 
 	/* deprecated thread usage histogram stats */
 	for (i = 0; i < 10; i++)

fs/nfsd/stats.h

@@ -29,11 +29,9 @@ enum {
 struct nfsd_stats {
 	struct percpu_counter	counter[NFSD_STATS_COUNTERS_NUM];
 
-	/* Protected by nfsd_mutex */
-	unsigned int	th_cnt;		/* number of available threads */
+	atomic_t	th_cnt;		/* number of available threads */
 };
 
-
 extern struct nfsd_stats	nfsdstats;
 
 extern struct svc_stat		nfsd_svcstats;