[PATCH v2] netfs, fscache: Prevent Oops in fscache_put_cache()

[Dan Carpenter]

This function dereferences "cache" and then checks if it's
IS_ERR_OR_NULL().  Check first, then dereference.

Fixes: 9549332df4ed ("fscache: Implement cache registration")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
v2: My first version introduced a race and a possible use after free.

 fs/netfs/fscache_cache.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/fscache_cache.c b/fs/netfs/fscache_cache.c
index d645f8b302a2..9397ed39b0b4 100644
--- a/fs/netfs/fscache_cache.c
+++ b/fs/netfs/fscache_cache.c
@@ -179,13 +179,14 @@ EXPORT_SYMBOL(fscache_acquire_cache);
 void fscache_put_cache(struct fscache_cache *cache,
 		       enum fscache_cache_trace where)
 {
-	unsigned int debug_id = cache->debug_id;
+	unsigned int debug_id;
 	bool zero;
 	int ref;
 
 	if (IS_ERR_OR_NULL(cache))
 		return;
 
+	debug_id = cache->debug_id;
 	zero = __refcount_dec_and_test(&cache->ref, &ref);
 	trace_fscache_cache(debug_id, ref - 1, where);
 
-- 
2.43.0