Index: sys/kern/subr_pool.c
===================================================================
RCS file: /cvsroot/src/sys/kern/subr_pool.c,v
retrieving revision 1.208
diff -p -u -r1.208 subr_pool.c
--- sys/kern/subr_pool.c	8 Jun 2017 04:00:01 -0000	1.208
+++ sys/kern/subr_pool.c	24 Oct 2017 14:46:21 -0000
@@ -1051,6 +1051,23 @@ pool_grow(struct pool *pp, int flags)
 {
 	struct pool_item_header *ph = NULL;
 	char *cp;
+	int error;
+
+	/*
+	 * If there's a pool_grow in progress, wait for it to complete
+	 * and try again from the top.
+	 */
+	if (pp->pr_flags & PR_GROWING) {
+		if (flags & PR_WAITOK) {
+			do {
+				cv_wait(&pp->pr_cv, &pp->pr_lock);
+			} while (pp->pr_flags & PR_GROWING);
+			return ERESTART;
+		} else {
+			return EWOULDBLOCK;
+		}
+	}
+	pp->pr_flags |= PR_GROWING;
 
 	mutex_exit(&pp->pr_lock);
 	cp = pool_allocator_alloc(pp, flags);
@@ -1062,13 +1079,25 @@ pool_grow(struct pool *pp, int flags)
 			pool_allocator_free(pp, cp);
 		}
 		mutex_enter(&pp->pr_lock);
-		return ENOMEM;
+		error = ENOMEM;
+		goto out;
 	}
 
 	mutex_enter(&pp->pr_lock);
 	pool_prime_page(pp, cp, ph);
 	pp->pr_npagealloc++;
-	return 0;
+	error = 0;
+
+out:
+	/*
+	 * If anyone was waiting for pool_grow, notify them that we
+	 * may have just done it.
+	 */
+	KASSERT(pp->pr_flags & PR_GROWING);
+	pp->pr_flags &= ~PR_GROWING;
+	cv_broadcast(&pp->pr_cv);
+
+	return error;
 }
 
 /*
Index: sys/sys/pool.h
===================================================================
RCS file: /cvsroot/src/sys/sys/pool.h,v
retrieving revision 1.79
diff -p -u -r1.79 pool.h
--- sys/sys/pool.h	29 Jul 2015 00:10:25 -0000	1.79
+++ sys/sys/pool.h	24 Oct 2017 14:46:21 -0000
@@ -147,6 +147,7 @@ struct pool {
 #define PR_NOTOUCH	0x400	/* don't use free items to keep internal state*/
 #define PR_NOALIGN	0x800	/* don't assume backend alignment */
 #define	PR_LARGECACHE	0x1000	/* use large cache groups */
+#define	PR_GROWING	0x2000	/* pool_grow in progress */
 
 	/*
 	 * `pr_lock' protects the pool's data structures when removing