On 01/26/2013 03:52 AM, Andrew Morton wrote: > On Tue, 22 Jan 2013 17:47:37 +0400 > Glauber Costa <glommer@xxxxxxxxxxxxx> wrote: > >> This patch is a preparatory work for later locking rework to get rid of >> big cgroup lock from memory controller code. > > Is this complete? From my reading, the patch is also a bugfix. It > prevents stale tunable values from getting installed into new children? > No, it is not a bug fix. This used to be all protected by the cgroup lock under the hood - we don't see it, but it is there from cgroup core. Yes, this is ugly. But it is one of the very problems this patchset is trying to get rid of =p >> The memory controller uses some tunables to adjust its operation. Those >> tunables are inherited from parent to children upon children >> intialization. For most of them, the value cannot be changed after the >> parent has a new children. >> >> cgroup core splits initialization in two phases: css_alloc and css_online. >> After css_alloc, the memory allocation and basic initialization are >> done. But the new group is not yet visible anywhere, not even for cgroup >> core code. It is only somewhere between css_alloc and css_online that it >> is inserted into the internal children lists. Copying tunable values in >> css_alloc will lead to inconsistent values: the children will copy the >> old parent values, that can change between the copy and the moment in >> which the groups is linked to any data structure that can indicate the >> presence of children. > > That describes the problem, but not the fix. Don't we need something > like "therefore move the propagation of tunables into the css_online > handler". > > What remains unclear is how we prevent races during the operation of > the css_online handler. Suppose mem_cgroup_css_online() is > mid-execution and userspace comes in and starts modifying the parent's > tunables? > At this point, the very same old cgroup_lock() - since it is still present. In a later patch, we will need the memcg mutex around the assignments. IOW, The figure looks a bit like: css_alloc() --> cgroup_internal_datastructure_update -> css_online() This is all protected by the cgroup_lock(). So at this point, wherever we do those assignments, we're safe. When we move to local locking, the situation changes. Assigning in css_alloc will mean that we'll have a non-locked window where the assignment is made, but the cgroup does not yet show up in the internal data structures - so the pertinence tests will fail and the tunable values will be allowed to change. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>