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>

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index a6c57334fa137749753ad5d3397c011bdae37ec3..72ad836e4fe05d7e5d9153918e5d8c1dacecc4a5 100644 (file)
--- 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;
+               }
        }
 }