[Devel] Re: [PATCH v5 3/3] cgroups: make procs file writable

[Ben Blum]

On Thu, Dec 16, 2010 at 12:26:03AM -0800, Andrew Morton wrote:
> Patches have gone a bit stale, sorry.  Refactoring in
> kernel/cgroup_freezer.c necessitates a refresh and retest please.

commit 53feb29767c29c877f9d47dcfe14211b5b0f7ebd changed a bunch of stuff
in kernel/cpuset.c to allocate nodemasks with NODEMASK_ALLOC (which
wraps kmalloc) instead of on the stack.

1. All these functions have 'void' return values, indicating that
   calling them them must not fail. Sure there are bailout cases, but no
   semblance of cross-function error propagation. Most importantly,
   cpuset_attach is a subsystem callback, which MUST not fail given the
   way it's used in cgroups, so relying on kmalloc is not safe.

2. I'm working on a patch series which needs to hold tasklist_lock
   across ss->attach callbacks (in cpuset_attach's "if (threadgroup)"
   case, this is how safe traversal of tsk->thread_group will be
   ensured), and kmalloc isn't allowed while holding a spin-lock. 

Why do we need heap-allocation here at all? In each case their scope is
exactly the function's scope, and neither the commit nor the surrounding
patch series give any explanation. I'd like to revert the patch if
possible.

cc'ing Miao Xie (author) and David Rientjes (acker).

-- Ben
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