knfsd: Replace lock_kernel with a mutex for nfsd thread startup/shutdown locking.

This removes the BKL from the RPC service creation codepath. The BKL
really isn't adequate for this job since some of this info needs
protection across sleeps.

Also, add some comments to try and clarify how the locking should work
and to make it clear that the BKL isn't necessary as long as there is
adequate locking between tasks when touching the svc_serv fields.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

1 files changed, 32 insertions, 13 deletions

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 941041f4b136..512bd04c6dda 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -53,11 +53,27 @@
 extern struct svc_program       nfsd_program;
 static void                     nfsd(struct svc_rqst *rqstp);
 struct timeval                  nfssvc_boot;
-       struct svc_serv          *nfsd_serv;
 static atomic_t                 nfsd_busy;
 static unsigned long            nfsd_last_call;
 static DEFINE_SPINLOCK(nfsd_call_lock);
 
+/*
+ * nfsd_mutex protects 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
+ *
+ * If (out side the lock) nfsd_serv is non-NULL, then it must point to a
+ * properly initialised 'struct svc_serv' with ->sv_nrthreads > 0. That number
+ * of nfsd threads must exist and each must listed in ->sp_all_threads in each
+ * entry of ->sv_pools[].
+ *
+ * Transitions of the thread count between zero and non-zero are of particular
+ * interest since the svc_serv needs to be created and initialized at that
+ * point, or freed.
+ */
+DEFINE_MUTEX(nfsd_mutex);
+struct svc_serv                 *nfsd_serv;
+
 #if defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL)
 static struct svc_stat  nfsd_acl_svcstats;
 static struct svc_version *     nfsd_acl_version[] = {
@@ -190,13 +206,14 @@ void nfsd_reset_versions(void)
         }
 }
 
+
 int nfsd_create_serv(void)
 {
         int err = 0;
-        lock_kernel();
+
+        WARN_ON(!mutex_is_locked(&nfsd_mutex));
         if (nfsd_serv) {
                 svc_get(nfsd_serv);
-                unlock_kernel();
                 return 0;
         }
         if (nfsd_max_blksize == 0) {
@@ -223,7 +240,7 @@ int nfsd_create_serv(void)
                                       nfsd, SIG_NOCLEAN, THIS_MODULE);
         if (nfsd_serv == NULL)
                 err = -ENOMEM;
-        unlock_kernel();
+
         do_gettimeofday(&nfssvc_boot);          /* record boot time */
         return err;
 }
@@ -282,6 +299,8 @@ int nfsd_set_nrthreads(int n, int *nthreads)
         int tot = 0;
         int err = 0;
 
+        WARN_ON(!mutex_is_locked(&nfsd_mutex));
+
         if (nfsd_serv == NULL || n <= 0)
                 return 0;
 
@@ -316,7 +335,6 @@ int nfsd_set_nrthreads(int n, int *nthreads)
                 nthreads[0] = 1;
 
         /* apply the new numbers */
-        lock_kernel();
         svc_get(nfsd_serv);
         for (i = 0; i < n; i++) {
                 err = svc_set_num_threads(nfsd_serv, &nfsd_serv->sv_pools[i],
@@ -325,7 +343,6 @@ int nfsd_set_nrthreads(int n, int *nthreads)
                         break;
         }
         svc_destroy(nfsd_serv);
-        unlock_kernel();
 
         return err;
 }
@@ -334,8 +351,8 @@ int
 nfsd_svc(unsigned short port, int nrservs)
 {
         int     error;
-        
-        lock_kernel();
+
+        mutex_lock(&nfsd_mutex);
         dprintk("nfsd: creating service\n");
         error = -EINVAL;
         if (nrservs <= 0)
@@ -363,7 +380,7 @@ nfsd_svc(unsigned short port, int nrservs)
  failure:
         svc_destroy(nfsd_serv);         /* Release server */
  out:
-        unlock_kernel();
+        mutex_unlock(&nfsd_mutex);
         return error;
 }
 
@@ -399,7 +416,7 @@ nfsd(struct svc_rqst *rqstp)
         sigset_t shutdown_mask, allowed_mask;
 
         /* Lock module and set up kernel thread */
-        lock_kernel();
+        mutex_lock(&nfsd_mutex);
         daemonize("nfsd");
 
         /* After daemonize() this kernel thread shares current->fs
@@ -417,11 +434,13 @@ nfsd(struct svc_rqst *rqstp)
         siginitsetinv(&shutdown_mask, SHUTDOWN_SIGS);
         siginitsetinv(&allowed_mask, ALLOWED_SIGS);
 
+
         nfsdstats.th_cnt++;
 
         rqstp->rq_task = current;
 
-        unlock_kernel();
+        mutex_unlock(&nfsd_mutex);
+
 
         /*
          * We want less throttling in balance_dirty_pages() so that nfs to
@@ -477,7 +496,7 @@ nfsd(struct svc_rqst *rqstp)
         /* Clear signals before calling svc_exit_thread() */
         flush_signals(current);
 
-        lock_kernel();
+        mutex_lock(&nfsd_mutex);
 
         nfsdstats.th_cnt --;
 
@@ -486,7 +505,7 @@ out:
         svc_exit_thread(rqstp);
 
         /* Release module */
-        unlock_kernel();
+        mutex_unlock(&nfsd_mutex);
         module_put_and_exit(0);
 }