[lxc-devel] [lxc/lxc] 308a6c: Revert "cgfsng: try to delete parent cgroups"

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/lxc/lxc
  Commit: 308a6c946d5226c043953e74f35f74c332e6764d
      https://github.com/lxc/lxc/commit/308a6c946d5226c043953e74f35f74c332e6764d
  Author: Christian Brauner <christian.brauner at ubuntu.com>
  Date:   2017-08-30 (Wed, 30 Aug 2017)

  Changed paths:
    M src/lxc/cgroups/cgfsng.c
    M src/lxc/confile.c

  Log Message:
  -----------
  Revert "cgfsng: try to delete parent cgroups"

This reverts commit 92c590ae1ea40bc094603ab49c20b785cc88bb1d.

Problem:

    Commit 92c590ae1ea40bc094603ab49c20b785cc88bb1d introduced the following
    behavior:

    > cgfsng: try to delete parent cgroups
    >
    > Say we have
    >
    >     lxc.uts.name = c1
    >     lxc.cgroup.dir = lxd/a/b/c
    >
    > the path for the container's cgroup would be
    >
    >     lxd/a/b/c/c1
    >
    > When the container is shutdown we should not just try to delete "c1" we
    > should also try to delete "c", "b", "a", and "lxd". This is to ensure
    > that we don't leave empty cgroups around thereby increasing the chance
    > that we run into trouble with cgroup limits. The algorithm for this isn't
    > too costly since we can simply stop walking upwards at the first rmdir()
    > failure.

    The algorithm employs recursive_destroy() which opens each directory
    specified in lxc.cgroup.dir and tries to delete each directory within that
    directory. For example, assume "/sys/fs/cgroup/memory/lxd/a/b/c" only
    contains the cgroup "c1" for container "c1". Assume that "c1" calls
    recursive_destroy() to cleanup it's cgroups. It will first delete "c1" and
    anything underneath it. This is perfectly fine since anything underneath
    that cgroup is under its control. The new algorithm will then tell it to
    "recurse upwards". So recursive_destroy() will try to delete
    "/sys/fs/cgroup/lxd/a/b/c" next. Now assume that a second container "c2"
    has "lxc.cgroup.dir = lxd/a/b/c" set in its config file and calls
    cgroup_create(). This will create the *empty* cgroup
    "/sys/fs/cgroup/memory/lxd/a/b/c/c2". Now assume that after having created
    "c2" container "c1"'s call to recursive_destroy() reaches
    "/sys/fs/cgroup/memory/lxd/a/b/c/c2" before it is populated. Then the
    cgroup "c2" will be removed. Now "c2" calls cgroup_enter() to enter its
    created cgroup. This will fail since c1 deleted the cgroup "c2". (As a
    sidenote: This is in the set of the few race conditions that are actually
    easy to describe.)

Possible Solution:

    Instead of calling recursive_destroy() on all cgroups specified in
    lxc.cgroup.dir we only call recursive_destroy() on the container's own
    cgroup "/sys/fs/cgroup/memory/lxd/a/b/c/c1". When we start to recurse
    upwards we only call unlinkat(AT_FDCWD, path, AT_REMOVEDIR). This should
    avoid the race described above. My argument is as follows. Assume that the
    container c1 has created the cgroup "/sys/fs/cgroup/lxd/a/b/c/c1" for
    itself. Now c1 calls cgroup_destroy(). First, recursive_destroy() will be
    called on the cgroup "c1" which will delete any emtpy cgroup directories
    underneath "c1" and finally "c1" itself. This is fine since everything
    under "c1" is the container's c1 sole property. Now container c1 will call
    unlinkat() on "/sys/fs/cgroup/memory/lxd/a/b/c/c1":
    - Assume that in the meantime container c2 has created the cgroup
      "/sys/fs/cgroup/memory/lxd/a/b/c/c2". Then c1's unlinkat() will fail.
      This will stop c1 from recursing upwards. So c2's cgroup_enter() call
      will find all its cgroups intact and well. unlinkat() will come with the
      appropriate in-kernel locking which will stop it from racing with
      mkdir().
    - There's still a subtle race left. c2 might be calling an implementation
      of mkdir -p to try and create e.g. the cgroup
      "/sys/fs/cgroup/memory/lxd/a/b". Let's assume "b" exists then c2 will
      receive EEXIST on "b" and move on to create "c". Let's further assume c1
      has already deleted "c". c1 will now be able to delete
      "/sys/fs/cgroup/memory/lxd/a/b/" and c2's call to create "c" will fail.

The latter subtle race makes me rethink this approach. For now we'll just leave
empty cgroups behind since I don't want to start locking stuff.

Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>


  Commit: cf7faeb345bf7af7036b15d89652f5c76039ec44
      https://github.com/lxc/lxc/commit/cf7faeb345bf7af7036b15d89652f5c76039ec44
  Author: Christian Brauner <christian.brauner at ubuntu.com>
  Date:   2017-08-30 (Wed, 30 Aug 2017)

  Changed paths:
    M src/lxc/confile.c

  Log Message:
  -----------
  confile: remove unnecessary cleanup code

set_config_string_item() already free()s before setting the new value.

Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>


  Commit: 70a498157a8d44ea95c77adb5096fa829dd54e1d
      https://github.com/lxc/lxc/commit/70a498157a8d44ea95c77adb5096fa829dd54e1d
  Author: Stéphane Graber <stgraber at stgraber.org>
  Date:   2017-08-30 (Wed, 30 Aug 2017)

  Changed paths:
    M src/lxc/cgroups/cgfsng.c

  Log Message:
  -----------
  Merge pull request #1769 from brauner/2017-08-30/improve_empty_cgroup_deletion

Revert "cgfsng: try to delete parent cgroups"


Compare: https://github.com/lxc/lxc/compare/2e02bbdb03c2...70a498157a8d