SUNRPC: change how svc threads are asked to exit.

svc threads are currently stopped using kthread_stop(). This requires
identifying a specific thread. However we don't care which thread
stops, just as long as one does.

So instead, set a flag in the svc_pool to say that a thread needs to
die, and have each thread check this flag instead of calling
kthread_should_stop(). The first thread to find and clear this flag
then moves towards exiting.

This removes an explicit dependency on sp_all_threads which will make a
future patch simpler.

Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/lockd/svc.c

···
 #include <linux/uio.h>
 #include <linux/smp.h>
 #include <linux/mutex.h>
-#include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/inetdevice.h>
 
···
 	 * The main request loop. We don't terminate until the last
 	 * NFS mount or NFS daemon has gone away.
 	 */
-	while (!kthread_should_stop()) {
+	while (!svc_thread_should_stop(rqstp)) {
 		/* update sv_maxconn if it has changed */
 		rqstp->rq_server->sv_maxconn = nlm_max_connections;
 
-		nlmsvc_retry_blocked();
+		nlmsvc_retry_blocked(rqstp);
 		svc_recv(rqstp);
 	}
 	if (nlmsvc_ops)

fs/lockd/svclock.c

···
 #include <linux/sunrpc/svc_xprt.h>
 #include <linux/lockd/nlm.h>
 #include <linux/lockd/lockd.h>
-#include <linux/kthread.h>
 #include <linux/exportfs.h>
 
 #define NLMDBG_FACILITY		NLMDBG_SVCLOCK
···
  * be retransmitted.
  */
 void
-nlmsvc_retry_blocked(void)
+nlmsvc_retry_blocked(struct svc_rqst *rqstp)
 {
 	unsigned long	timeout = MAX_SCHEDULE_TIMEOUT;
 	struct nlm_block *block;
 
 	spin_lock(&nlm_blocked_lock);
-	while (!list_empty(&nlm_blocked) && !kthread_should_stop()) {
+	while (!list_empty(&nlm_blocked) && !svc_thread_should_stop(rqstp)) {
 		block = list_entry(nlm_blocked.next, struct nlm_block, b_list);
 
 		if (block->b_when == NLM_NEVER)

fs/nfs/callback.c

···
 
 	set_freezable();
 
-	while (!kthread_should_stop())
+	while (!svc_thread_should_stop(rqstp))
 		svc_recv(rqstp);
 
 	svc_exit_thread(rqstp);

fs/nfsd/nfs4proc.c

···
  * setup a work entry in the ssc delayed unmount list.
  */
 static __be32 nfsd4_ssc_setup_dul(struct nfsd_net *nn, char *ipaddr,
-				  struct nfsd4_ssc_umount_item **nsui)
+				  struct nfsd4_ssc_umount_item **nsui,
+				  struct svc_rqst *rqstp)
 {
 	struct nfsd4_ssc_umount_item *ni = NULL;
 	struct nfsd4_ssc_umount_item *work = NULL;
···
 			spin_unlock(&nn->nfsd_ssc_lock);
 
 			/* allow 20secs for mount/unmount for now - revisit */
-			if (kthread_should_stop() ||
+			if (svc_thread_should_stop(rqstp) ||
 					(schedule_timeout(20*HZ) == 0)) {
 				finish_wait(&nn->nfsd_ssc_waitq, &wait);
 				kfree(work);
···
 		goto out_free_rawdata;
 	snprintf(dev_name, len + 5, "%s%s%s:/", startsep, ipaddr, endsep);
 
-	status = nfsd4_ssc_setup_dul(nn, ipaddr, nsui);
+	status = nfsd4_ssc_setup_dul(nn, ipaddr, nsui, rqstp);
 	if (status)
 		goto out_free_devname;
 	if ((*nsui)->nsui_vfsmount)
···
 	if (bytes_total == 0)
 		bytes_total = ULLONG_MAX;
 	do {
+		/* Only async copies can be stopped here */
 		if (kthread_should_stop())
 			break;
 		bytes_copied = nfsd_copy_file_range(src, src_pos, dst, dst_pos,

fs/nfsd/nfssvc.c

···
 	/*
 	 * The main request loop
 	 */
-	while (!kthread_should_stop()) {
+	while (!svc_thread_should_stop(rqstp)) {
 		/* Update sv_maxconn if it has changed */
 		rqstp->rq_server->sv_maxconn = nn->max_connections;
 

include/linux/lockd/lockd.h

···
 			struct nlm_host *, struct nlm_lock *,
 			struct nlm_lock *, struct nlm_cookie *);
 __be32		  nlmsvc_cancel_blocked(struct net *net, struct nlm_file *, struct nlm_lock *);
-void		  nlmsvc_retry_blocked(void);
+void		  nlmsvc_retry_blocked(struct svc_rqst *rqstp);
 void		  nlmsvc_traverse_blocks(struct nlm_host *, struct nlm_file *,
 					nlm_host_match_fn_t match);
 void		  nlmsvc_grant_reply(struct nlm_cookie *, __be32);

include/linux/sunrpc/svc.h

···
 enum {
 	SP_TASK_PENDING,	/* still work to do even if no xprt is queued */
 	SP_CONGESTED,		/* all threads are busy, none idle */
+	SP_NEED_VICTIM,		/* One thread needs to agree to exit */
+	SP_VICTIM_REMAINS,	/* One thread needs to actually exit */
 };
 
 
···
 	RQ_DROPME,		/* drop current reply */
 	RQ_SPLICE_OK,		/* turned off in gss privacy to prevent
 				 * encrypting page cache pages */
-	RQ_VICTIM,		/* about to be shut down */
+	RQ_VICTIM,		/* Have agreed to shut down */
 	RQ_BUSY,		/* request is busy */
 	RQ_DATA,		/* request has data */
 };
···
 static inline struct sockaddr *svc_daddr(const struct svc_rqst *rqst)
 {
 	return (struct sockaddr *) &rqst->rq_daddr;
+}
+
+/**
+ * svc_thread_should_stop - check if this thread should stop
+ * @rqstp: the thread that might need to stop
+ *
+ * To stop an svc thread, the pool flags SP_NEED_VICTIM and SP_VICTIM_REMAINS
+ * are set.  The first thread which sees SP_NEED_VICTIM clears it, becoming
+ * the victim using this function.  It should then promptly call
+ * svc_exit_thread() to complete the process, clearing SP_VICTIM_REMAINS
+ * so the task waiting for a thread to exit can wake and continue.
+ *
+ * Return values:
+ *   %true: caller should invoke svc_exit_thread()
+ *   %false: caller should do nothing
+ */
+static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
+{
+	if (test_and_clear_bit(SP_NEED_VICTIM, &rqstp->rq_pool->sp_flags))
+		set_bit(RQ_VICTIM, &rqstp->rq_flags);
+
+	return test_bit(RQ_VICTIM, &rqstp->rq_flags);
 }
 
 struct svc_deferred_req {

net/sunrpc/svc.c

···
 	return pool ? pool : &serv->sv_pools[(*state)++ % serv->sv_nrpools];
 }
 
-static struct task_struct *
+static struct svc_pool *
 svc_pool_victim(struct svc_serv *serv, struct svc_pool *pool, unsigned int *state)
 {
 	unsigned int i;
-	struct task_struct *task = NULL;
 
 	if (pool != NULL) {
 		spin_lock_bh(&pool->sp_lock);
+		if (pool->sp_nrthreads)
+			goto found_pool;
+		spin_unlock_bh(&pool->sp_lock);
+		return NULL;
 	} else {
 		for (i = 0; i < serv->sv_nrpools; i++) {
 			pool = &serv->sv_pools[--(*state) % serv->sv_nrpools];
 			spin_lock_bh(&pool->sp_lock);
-			if (!list_empty(&pool->sp_all_threads))
+			if (pool->sp_nrthreads)
 				goto found_pool;
 			spin_unlock_bh(&pool->sp_lock);
 		}
···
 	}
 
 found_pool:
-	if (!list_empty(&pool->sp_all_threads)) {
-		struct svc_rqst *rqstp;
-
-		rqstp = list_entry(pool->sp_all_threads.next, struct svc_rqst, rq_all);
-		set_bit(RQ_VICTIM, &rqstp->rq_flags);
-		list_del_rcu(&rqstp->rq_all);
-		task = rqstp->rq_task;
-	}
+	set_bit(SP_VICTIM_REMAINS, &pool->sp_flags);
+	set_bit(SP_NEED_VICTIM, &pool->sp_flags);
 	spin_unlock_bh(&pool->sp_lock);
-	return task;
+	return pool;
 }
 
 static int
···
 static int
 svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 {
-	struct svc_rqst	*rqstp;
-	struct task_struct *task;
 	unsigned int state = serv->sv_nrthreads-1;
+	struct svc_pool *victim;
 
 	do {
-		task = svc_pool_victim(serv, pool, &state);
-		if (task == NULL)
+		victim = svc_pool_victim(serv, pool, &state);
+		if (!victim)
 			break;
-		rqstp = kthread_data(task);
-		/* Did we lose a race to svo_function threadfn? */
-		if (kthread_stop(task) == -EINTR)
-			svc_exit_thread(rqstp);
+		svc_pool_wake_idle_thread(victim);
+		wait_on_bit(&victim->sp_flags, SP_VICTIM_REMAINS,
+			    TASK_IDLE);
 		nrservs++;
 	} while (nrservs < 0);
 	return 0;
···
 
 	spin_lock_bh(&pool->sp_lock);
 	pool->sp_nrthreads--;
-	if (!test_and_set_bit(RQ_VICTIM, &rqstp->rq_flags))
-		list_del_rcu(&rqstp->rq_all);
+	list_del_rcu(&rqstp->rq_all);
 	spin_unlock_bh(&pool->sp_lock);
 
 	spin_lock_bh(&serv->sv_lock);
···
 	svc_rqst_free(rqstp);
 
 	svc_put(serv);
+	/* That svc_put() cannot be the last, because the thread
+	 * waiting for SP_VICTIM_REMAINS to clear must hold
+	 * a reference. So it is still safe to access pool.
+	 */
+	clear_and_wake_up_bit(SP_VICTIM_REMAINS, &pool->sp_flags);
 }
 EXPORT_SYMBOL_GPL(svc_exit_thread);
 

net/sunrpc/svc_xprt.c

···
 #include <linux/sched/mm.h>
 #include <linux/errno.h>
 #include <linux/freezer.h>
-#include <linux/kthread.h>
 #include <linux/slab.h>
 #include <net/sock.h>
 #include <linux/sunrpc/addr.h>
···
 			continue;
 
 		set_current_state(TASK_IDLE);
-		if (kthread_should_stop()) {
+		if (svc_thread_should_stop(rqstp)) {
 			set_current_state(TASK_RUNNING);
 			return false;
 		}
···
 		return false;
 
 	/* are we shutting down? */
-	if (kthread_should_stop())
+	if (svc_thread_should_stop(rqstp))
 		return false;
 
 	/* are we freezing? */
···
 
 	clear_bit(SP_TASK_PENDING, &pool->sp_flags);
 
-	if (kthread_should_stop())
+	if (svc_thread_should_stop(rqstp))
 		return;
 
 	rqstp->rq_xprt = svc_xprt_dequeue(pool);