- Automatically Locate RCU Abuses
- Inline __rcu_read_lock()
- Add kmem_cache_free_rcu()
- Validate RCU Algorithms
- Automate Testing of RCU CPU Stall Warnings
- Port RCU's KVM Scripts
- Miscellaneous Fixes to RCU
Structured Deferral: Synchronization via Procrastination (Conceptual overview.)
What Is RCU? (Another conceptual overview, but in slide form.)
The RCU API, 2010 Edition (API overview.)
And last but not least, the location of the -rcu tree: git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git. The commits intended for the next merge window are at rcu/next, and the latest development is at rcu/dev.
Automatically Locate RCU Abuses
Uses of rcu_dereference() (and friends) whose return value is never dereferenced should be converted to rcu_access_pointer(). For example, "if (rcu_dereference(p) == NULL)" should be converted to "if (rcu_access_pointer(p) == NULL)". Perhaps better, as Josh Triplett suggests, would be to create something like rcu_pointer_eq() to compare an RCU-protected pointer to some other pointer, including of course a NULL pointer. The friends of rcu_dereference() include rcu_dereference_check(), rcu_dereference_bh(), rcu_dereference_bh_check(), rcu_dereference_raw(), rcu_dereference_sched(), rcu_dereference_sched_check(), srcu_dereference(), and srcu_dereference_check(). Note that comparison to non-NULL pointers are subject to a more complex set of rules, so it is best to start with comparisons to NULL pointers.
Dereferencing an RCU-protected pointer outside of the RCU read-side critical section that it was obtained in, unless it was protected by a reference count, lock, or some such before exiting the RCU read-side critical section.
The "_ _rcu_read_lock()" function is currently located in kernel/rcu/update.c is an ideal candidate for inlining. Except for one thing, namely that exporting include/linux/sched.h's task_struct to every file that needs "_ _rcu_read_lock()" results in very ugly circular include-file dependencies. The goal is therefore to get rid of these circular dependencies. There are two known approaches:
Split the data structure definitions out into a parallel set of include files. This approach is in some sense cleaner than replacing inline functions with C-preprocessor macros, but requires careful handling of conflicting patches that are affected by the wholesale changes.
Add a kmem_cache_free_rcu() function that invokes kmem_cache_free() on the referenced structure after an RCU grace period elapses. Use kmem_cache_free_rcu() to remove the various mini-functions that look like the following:
There are some strict constraints. First, this change is not permitted to change the size of the rcu_head structure. One way to avoid such a size change is to create an RCU-protected hash table that contains entries for slabs that have ever been passed to kmem_cache_free_rcu(). Each entry in this hash table would need to contain the offset of the rcu_head structure within the enclosing structure.
Note that a given structure could in theory have multiple rcu_head structures, but this is rare in practice. One acceptable option is to complain and leak callbacks in this case. Another acceptable option is to correctly handle this case. Approaches that result in inexplicable crashes need not apply.
Validate RCU Algorithms
Use tools such as Promela/spin or goto-cc/goto-instrument/satabs to validate (or invalidate, as the case may be) some of RCU's more involved algorithms. In the case of invalidation, fixes would of course be quite welcome.
Automate Testing of RCU CPU Stall Warnings
One of rcutorture's many shortcomings is that it does not do much to automatically test RCU CPU stall warnings, which are described in Documentation/RCU/stallwarn.txt. Add automated testing to either kernel/rcu/torture.c or make some other tool that creates CPU stalls and verifies that RCU complains appropriately.
Port RCU's KVM Scripts
There is a set of scripts in tools/testing/selftests/rcutorture that automatically build, boot, and test kernels for a number of RCU configurations. An initial version of these is in mainline, but the current version may be found at git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git on branch rcu/dev.
Miscellaneous Fixes to RCU
Exercise RCU's debugfs tracing, and fix any bugs in either the implementation or the documentation. The debugfs tracing is normally located in /sys/kernel/debug/rcu, and the documentation is in Documentation/RCU/trace.txt.