On Mon, Aug 29, 2011 at 12:22:02AM -0700, Ying Han wrote:

fix hierarchy_walk() to hold a reference to first mem_cgroup

The first mem_cgroup returned from hierarchy_walk() is used to
terminate a round-trip. However there is no reference hold on
that which the first could be removed during the walking. The
patch including the following change:

1. hold a reference on the first mem_cgroup during the walk.
2. rename the variable "root" to "target", which we found using
"root" is confusing in this content with root_mem_cgroup. better
naming is welcomed.

Thanks for the report.

This was actually not the only case that could lead to overlong (not
necessarily endless) looping.

With several scanning threads, a single thread may not encounter its
first cgroup again for a long time, as the other threads would visit
it.

I changed this to use scan generations.  Restarting the scan from id 0
starts the next scan generation.  The iteration function returns NULL
if the generation changed since a loop was started.

This way, iterators can reliably detect whether they should call it
quits without any requirements for previously encountered memcgs.

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