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Subject[PATCH 11/13] cpuset: pin down cpus and mems while a task is being attached
Date
cpuset is scheduled to be decoupled from cgroup_lock which will make
configuration updates race with task migration. Any config update
will be allowed to happen between ->can_attach() and ->attach(). If
such config update removes either all cpus or mems, by the time
->attach() is called, the condition verified by ->can_attach(), that
the cpuset is capable of hosting the tasks, is no longer true.

This patch adds cpuset->attach_in_progress which is incremented from
->can_attach() and decremented when the attach operation finishes
either successfully or not. validate_change() treats cpusets w/
non-zero ->attach_in_progress like cpusets w/ tasks and refuses to
remove all cpus or mems from it.

This currently doesn't make any functional difference as everything is
protected by cgroup_mutex but enables decoupling the locking.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/cpuset.c | 28 ++++++++++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 3558250..68a0906 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -91,6 +91,12 @@ struct cpuset {

struct fmeter fmeter; /* memory_pressure filter */

+ /*
+ * Tasks are being attached to this cpuset. Used to prevent
+ * zeroing cpus/mems_allowed between ->can_attach() and ->attach().
+ */
+ int attach_in_progress;
+
/* partition number for rebuild_sched_domains() */
int pn;

@@ -468,9 +474,12 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
goto out;
}

- /* Cpusets with tasks can't have empty cpus_allowed or mems_allowed */
+ /*
+ * Cpusets with tasks - existing or newly being attached - can't
+ * have empty cpus_allowed or mems_allowed.
+ */
ret = -ENOSPC;
- if (cgroup_task_count(cur->css.cgroup) &&
+ if ((cgroup_task_count(cur->css.cgroup) || cur->attach_in_progress) &&
(cpumask_empty(trial->cpus_allowed) ||
nodes_empty(trial->mems_allowed)))
goto out;
@@ -1386,9 +1395,21 @@ static int cpuset_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
return ret;
}

+ /*
+ * Mark attach is in progress. This makes validate_change() fail
+ * changes which zero cpus/mems_allowed.
+ */
+ cs->attach_in_progress++;
+
return 0;
}

+static void cpuset_cancel_attach(struct cgroup *cgrp,
+ struct cgroup_taskset *tset)
+{
+ cgroup_cs(cgrp)->attach_in_progress--;
+}
+
static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
{
/* static bufs protected by cgroup_mutex */
@@ -1435,6 +1456,8 @@ static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
&cpuset_attach_nodemask_to);
mmput(mm);
}
+
+ cs->attach_in_progress--;
}

/* The various types of files and directories in a cpuset file system */
@@ -1902,6 +1925,7 @@ struct cgroup_subsys cpuset_subsys = {
.css_offline = cpuset_css_offline,
.css_free = cpuset_css_free,
.can_attach = cpuset_can_attach,
+ .cancel_attach = cpuset_cancel_attach,
.attach = cpuset_attach,
.subsys_id = cpuset_subsys_id,
.base_cftypes = files,
--
1.7.11.7


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