gaining momentum

I spent some time today looking through kernel/cgroup.c (and a bit of cgroup.h), focusing on the uses of the subsys[] array - the one that's currently initialized and link-time and will need to be redone for dynamic loading - and thinking about how to change it.

For one thing, cgroup_init_subsys() will need to no longer be marked as __init, and will need to be concurrency-safe (against itself, too). I think that no matter what data structure I end up using for the subsystem list, it will be guarded by a rwlock (or rwsem. one is a low-level lock, the other isn't). Aside from that, the only other challenge there appears to be the fork/exit callbacks logic, which I think can be for the most part disregarded - or thought about very briefly at least - since currently no subsystem even uses the callbacks.

As for the subsys[] array, the question of how to make it support new guys appearing is a difficult one. At first I thought I could make it a list, with list_head structs in each subsystem's struct - which might still be possible... but looking through all the uses of it, there will prove to be some fiddly bits. Namely, there are other data structures that rely on the list of subsystems being a fixed-length array - cgroup->subsys[] and css_set->subsys[], which is a list of cgroup_subsys_state objects (good naming there, guys), and also the use of template[] in find_existing_css_set (which I used over the summer!) all rely on matching up with the global subsys[] array. Additionally, struct cgroupfs_root has a pair of fields (unsigned long) called subsys_bits and actual_subsys_bits, which keep track of which subsystems are or want to be attached. So, thoughts for this are either:

1. figure out some way to do a dynamic list for subsys[] and its corresponding things, which will involve possibly fiddly uses of kmalloc() (with accompanying fail cases) and/or relying on cgroup_mutex and throwing another list_head in the subsys structs somewhere. also, replacing subsys_bits with something more suitable.
2. simply set CGROUP_SUBSYS_COUNT to sizeof(subsys_bits)*8 (i.e., maximum number of subsystems at a time is the number of bits in the thing field) and let the array have null slots in it. this seems a lot easier, but is avoiding the design problem. on the other hand, they designed clone_flags to have 32 possible settings, and they ran out of those recently, so...

I hope to spend a good chunk of tomorrow coding.