commit 070cde3e1df228e6d4238398aab0bd8917b63555
Author: Mateusz Guzik <mjg@FreeBSD.org>
Date:   Mon Mar 29 21:17:57 2021 +0200

    cache: fix resizing in face of lockless lookup

diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
index 8cae0260cbf0..7f4644970920 100644
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -455,6 +455,9 @@ static long cache_lock_vnodes_cel_3_failures;
 DEBUGNODE_ULONG(vnodes_cel_3_failures, cache_lock_vnodes_cel_3_failures,
     "Number of times 3-way vnode locking failed");
 
+static void cache_fplookup_lockout(void);
+static void cache_fplookup_restore(void);
+
 static void cache_zap_locked(struct namecache *ncp);
 static int vn_fullpath_hardlink(struct nameidata *ndp, char **retbuf,
     char **freebuf, size_t *buflen);
@@ -2575,6 +2578,7 @@ cache_changesize(u_long newmaxvnodes)
 	 * None of the namecache entries in the table can be removed
 	 * because to do so, they have to be removed from the hash table.
 	 */
+	cache_fplookup_lockout();
 	cache_lock_all_vnodes();
 	cache_lock_all_buckets();
 	old_nchashtbl = nchashtbl;
@@ -2593,6 +2597,7 @@ cache_changesize(u_long newmaxvnodes)
 	cache_recalc_neg_min(ncnegminpct);
 	cache_unlock_all_buckets();
 	cache_unlock_all_vnodes();
+	cache_fplookup_restore();
 	ncfreetbl(old_nchashtbl);
 }
 
@@ -3666,6 +3671,33 @@ syscal_vfs_cache_fast_lookup(SYSCTL_HANDLER_ARGS)
 SYSCTL_PROC(_vfs, OID_AUTO, cache_fast_lookup, CTLTYPE_INT|CTLFLAG_RW|CTLFLAG_MPSAFE,
     &cache_fast_lookup, 0, syscal_vfs_cache_fast_lookup, "IU", "");
 
+/*
+ * Disable lockless lookup and observe all CPUs not executing it.
+ *
+ * Used when resizing the hash table.
+ *
+ * TODO: no provisions are made to handle tweaking of the knob at the same time
+ */
+static void
+cache_fplookup_lockout(void)
+{
+	bool on;
+
+	on = atomic_load_char(&cache_fast_lookup_enabled);
+	if (on) {
+		atomic_store_char(&cache_fast_lookup_enabled, false);
+		atomic_thread_fence_rel();
+		quiesce_all_critical();
+	}
+}
+
+static void
+cache_fplookup_restore(void)
+{
+
+	cache_fast_lookup_enabled_recalc();
+}
+
 /*
  * Components of nameidata (or objects it can point to) which may
  * need restoring in case fast path lookup fails.