10.3.2.6 RCU is a Way of Providing Type-Safe Memory
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10.3.2.6 RCU is a Way of Providing Type-Safe Memory

A number of lockless algorithms do not require that a given data element keep the same identity through a given RCU read-side critical section referencing it--but only if that data element retains the same type. In other words, these lockless algorithms can tolerate a given data element being freed and reallocated as the same type of structure while they are referencing it, but must prohibit a change in type. This guarantee, called ``type-safe memory'' in academic literature [GC96], is weaker than the existence guarantees in the previous section, and is therefore quite a bit harder to work with. Type-safe memory algorithms in the Linux kernel make use of slab caches, specially marking these caches with SLAB_DESTROY_BY_RCU so that RCU is used when returning a freed-up slab to system memory. This use of RCU guarantees that any in-use element of such a slab will remain in that slab, thus retaining its type, for the duration of any pre-existing RCU read-side critical sections.

Quick Quiz 10.20: But what if there is an arbitrarily long series of RCU read-side critical sections in multiple threads, so that at any point in time there is at least one thread in the system executing in an RCU read-side critical section? Wouldn't that prevent any data from a SLAB_DESTROY_BY_RCU slab ever being returned to the system, possibly resulting in OOM events? End Quick Quiz

These algorithms typically use a validation step that checks to make sure that the newly referenced data structure really is the one that was requested [LS86, Section 2.5]. These validation checks require that portions of the data structure remain untouched by the free-reallocate process. Such validation checks are usually very hard to get right, and can hide subtle and difficult bugs.

Therefore, although type-safety-based lockless algorithms can be extremely helpful in a very few difficult situations, you should instead use existence guarantees where possible. Simpler is after all almost always better!