From 8755ade683241e8c6b8fe8d22d0ae35041a3dc51 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 16 Oct 2012 15:03:14 -0700 Subject: [PATCH] cgroup_freezer: allow moving tasks in and out of a frozen cgroup cgroup_freezer is one of the few users of cgroup_subsys->can_attach() and uses it to prevent tasks from being migrated into or out of a frozen cgroup. This makes cgroup_freezer cumbersome to use especially when co-mounted with other controllers. ->can_attach() is problematic in general as it can make co-mounting multiple cgroups difficult - migrating tasks may fail for reasons completely irrelevant for other controllers. freezer_can_attach() in particular is more problematic because it messes with cgroup internal locking to ensure that the state verification performed at freezer_can_attach() stays valid until migration is complete. This patch replaces freezer_can_attach() with freezer_attach() so that tasks are always allowed to migrate - they are nudged into the conforming state from freezer_attach(). This means that there can be tasks which are being migrated which don't conform to the current cgroup_freezer state until freezer_attach() is complete. Under the current locking scheme, the only such place is freezer_fork() which is updated to handle such window. While this patch doesn't remove the use of internal cgroup locking from freezer_read/write() paths, it removes the requirement to keep the freezer state constant while migrating and enables such change. Note that this creates a userland visible behavior change - FROZEN cgroup can no longer be used to lock migrations in and out of the cgroup. This behavior change is intended. I don't think the feature is necessary - userland should coordinate accesses to cgroup fs anyway - and even if the feature is needed cgroup_freezer is the completely wrong place to implement it. Signed-off-by: Tejun Heo LKML-Reference: <1350426526-14254-1-git-send-email-tj@kernel.org> Cc: Matt Helsley Cc: Oleg Nesterov Cc: Rafael J. Wysocki Cc: Li Zefan --- Reading git-format-patch failed