svcrpc: ensure cache_check caller sees updated entry

Supposes cache_check runs simultaneously with an update on a different
CPU:

	cache_check			task doing update
	^^^^^^^^^^^			^^^^^^^^^^^^^^^^^

	1. test for CACHE_VALID		1'. set entry->data
	   & !CACHE_NEGATIVE

	2. use entry->data		2'. set CACHE_VALID

If the two memory writes performed in step 1' and 2' appear misordered
with respect to the reads in step 1 and 2, then the caller could get
stale data at step 2 even though it saw CACHE_VALID set on the cache
entry.

Add memory barriers to prevent this.

Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index a6c57334fa13..72ad836e4fe0 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -128,6 +128,7 @@ static void cache_fresh_locked(struct cache_head *head, time_t expiry)
 {
         head->expiry_time = expiry;
         head->last_refresh = seconds_since_boot();
+        smp_wmb(); /* paired with smp_rmb() in cache_is_valid() */
         set_bit(CACHE_VALID, &head->flags);
 }
 
@@ -208,8 +209,16 @@ static inline int cache_is_valid(struct cache_detail *detail, struct cache_head
                 /* entry is valid */
                 if (test_bit(CACHE_NEGATIVE, &h->flags))
                         return -ENOENT;
-                else
+                else {
+                        /*
+                         * In combination with write barrier in
+                         * sunrpc_cache_update, ensures that anyone
+                         * using the cache entry after this sees the
+                         * updated contents:
+                         */
+                        smp_rmb();
                         return 0;
+                }
         }
 }