Patch "cpuset: consider dying css as offline" has been added to the 3.18-stable tree

Patch "cpuset: consider dying css as offline" has been added to the 3.18-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    cpuset: consider dying css as offline

to the 3.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cpuset-consider-dying-css-as-offline.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 41c25707d21716826e3c1f60967f5550610ec1c9 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 24 May 2017 12:03:48 -0400
Subject: cpuset: consider dying css as offline

From: Tejun Heo <tj@kernel.org>

commit 41c25707d21716826e3c1f60967f5550610ec1c9 upstream.

In most cases, a cgroup controller don't care about the liftimes of
cgroups.  For the controller, a css becomes online when ->css_online()
is called on it and offline when ->css_offline() is called.

However, cpuset is special in that the user interface it exposes cares
whether certain cgroups exist or not.  Combined with the RCU delay
between cgroup removal and css offlining, this can lead to user
visible behavior oddities where operations which should succeed after
cgroup removals fail for some time period.  The effects of cgroup
removals are delayed when seen from userland.

This patch adds css_is_dying() which tests whether offline is pending
and updates is_cpuset_online() so that the function returns false also
while offline is pending.  This gets rid of the userland visible
delays.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Link: http://lkml.kernel.org/r/327ca1f5-7957-fbb9-9e5f-9ba149d40ba2@oracle.com
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 include/linux/cgroup.h |   20 ++++++++++++++++++++
 kernel/cpuset.c        |    4 ++--
 2 files changed, 22 insertions(+), 2 deletions(-)

--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -148,6 +148,26 @@ static inline bool css_tryget_online(str
 }
 
 /**
+ * css_is_dying - test whether the specified css is dying
+ * @css: target css
+ *
+ * Test whether @css is in the process of offlining or already offline.  In
+ * most cases, ->css_online() and ->css_offline() callbacks should be
+ * enough; however, the actual offline operations are RCU delayed and this
+ * test returns %true also when @css is scheduled to be offlined.
+ *
+ * This is useful, for example, when the use case requires synchronous
+ * behavior with respect to cgroup removal.  cgroup removal schedules css
+ * offlining but the css can seem alive while the operation is being
+ * delayed.  If the delay affects user visible semantics, this test can be
+ * used to resolve the situation.
+ */
+static inline bool css_is_dying(struct cgroup_subsys_state *css)
+{
+	return !(css->flags & CSS_NO_REF) && percpu_ref_is_dying(&css->refcnt);
+}
+
+/**
  * css_put - put a css reference
  * @css: target css
  *
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -174,9 +174,9 @@ typedef enum {
 } cpuset_flagbits_t;
 
 /* convenient tests for these bits */
-static inline bool is_cpuset_online(const struct cpuset *cs)
+static inline bool is_cpuset_online(struct cpuset *cs)
 {
-	return test_bit(CS_ONLINE, &cs->flags);
+	return test_bit(CS_ONLINE, &cs->flags) && !css_is_dying(&cs->css);
 }
 
 static inline int is_cpu_exclusive(const struct cpuset *cs)


Patches currently in stable-queue which might be from tj@kernel.org are

queue-3.18/cpuset-consider-dying-css-as-offline.patch