svcrpc: ensure cache_check caller sees updated entry (fdef7aa5) · Commits · e / devices / android_kernel_samsung_universal8895

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>

net/sunrpc/cache.c

+10 −1

Original line number	Diff line number	Diff line
		@@ -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);
		}

		@@ -208,10 +209,18 @@ 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;
		}
		}
		}

		static int try_to_negate_entry(struct cache_detail detail, struct cache_head h)
		{