From: J. Bruce Fields Date: Tue, 4 Jan 2011 19:12:47 +0000 (-0500) Subject: svcrpc: ensure cache_check caller sees updated entry X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=fdef7aa5d4020fd94ffcbf0078d6bd9e5a111e19;p=openwrt%2Fstaging%2Fblogic.git 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 Signed-off-by: J. Bruce Fields --- 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; + } } }