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An early init subsystem that attempts to make use of rstat can lead to
failures during early boot. The reason for this is the timing in which the
css's of the root cgroup have css_online() invoked on them. At the point of
this call, there is a stated assumption that a cgroup has "successfully
completed all allocations" [0]. An example of a subsystem that relies on
the previously mentioned assumption [0] is the memory subsystem. Within its
implementation of css_online(), work is queued to asynchronously begin
flushing via rstat. In the early init path for a given subsystem, having
rstat enabled leads to this sequence:
cgroup_init_early()
for_each_subsys(ss, ssid)
if (ss->early_init)
cgroup_init_subsys(ss, true)
cgroup_init_subsys(ss, early_init)
css = ss->css_alloc(...)
init_and_link_css(css, ss, ...)
...
online_css(css)
online_css(css)
ss = css->ss
ss->css_online(css)
Continuing to use the memory subsystem as an example, the issue with this
sequence is that css_rstat_init() has not been called yet. This means there
is now a race between the pending async work to flush rstat and the call to
css_rstat_init(). So a flush can occur within the given cgroup while the
rstat fields are not initialized.
Since we are in the early init phase, the rstat fields cannot be
initialized because they require per-cpu allocations. So it's not possible
to have css_rstat_init() called early enough (before online_css()). This
patch treats the combination of early init and rstat the same as as other
invalid conditions.
[0] Documentation/admin-guide/cgroup-v1/cgroups.rst (section: css_online)
Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.