From: Manfred Spraul <manfred@colorfullife.com>

readdir on /proc has two problems: reading all entries is O(N^2), and
entries are overlooked if tasks die in the middle of readdir: the readdir
implementation remembers the offset into the task list, and if a task
(actually: process) that was returned by previous readdir calls exits, then
a random entry is dropped.

The attached patch fixes the O(N^2) by using f_version to store the pid of
the task that should be returned next.  This speeds up reading /proc to
O(N).  Additionally, it mitigates the effects of dying tasks: Tasks are
skipped only if the task whose pid is stored in f_version exits, all other
task deaths have no effect.  Unfortunately the code has a bad worst case
behavior: if the targeted task exits and a new task with the same pid is
created, then all entries in the task list between old and new position are
dropped.  This should be rare.