doc: SLAB_TYPESAFE_BY_RCU uses cannot rely on spinlocks

Because the SLAB_TYPESAFE_BY_RCU code does not zero pages that are
to be broken up into slabs, the memory returned by kmem_cache_alloc()
must be fully initialized, including any spinlocks included in the newly
allocated structure.  This means that readers attempting to look up an
SLAB_TYPESAFE_BY_RCU object must use a reference-counting approach.
A spinlock may be acquired only after a reference is obtained, which
prevents that object from being passed to kmem_struct_free(), but only
while that reference continues to be held.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/whatisRCU.rst

@@ -915,13 +915,18 @@ which an RCU reference is held include:
 The understanding that RCU provides a reference that only prevents a
 change of type is particularly visible with objects allocated from a
 slab cache marked ``SLAB_TYPESAFE_BY_RCU``.  RCU operations may yield a
-reference to an object from such a cache that has been concurrently
-freed and the memory reallocated to a completely different object,
-though of the same type.  In this case RCU doesn't even protect the
-identity of the object from changing, only its type.  So the object
-found may not be the one expected, but it will be one where it is safe
-to take a reference or spinlock and then confirm that the identity
-matches the expectations.
+reference to an object from such a cache that has been concurrently freed
+and the memory reallocated to a completely different object, though of
+the same type.  In this case RCU doesn't even protect the identity of the
+object from changing, only its type.  So the object found may not be the
+one expected, but it will be one where it is safe to take a reference
+(and then potentially acquiring a spinlock), allowing subsequent code
+to check whether the identity matches expectations.  It is tempting
+to simply acquire the spinlock without first taking the reference, but
+unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be
+initialized after each and every call to kmem_cache_alloc(), which renders
+reference-free spinlock acquisition completely unsafe.  Therefore, when
+using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter.
 
 With traditional reference counting -- such as that implemented by the
 kref library in Linux -- there is typically code that runs when the last