[PATCH v3] ipc: use a work queue to free_ipc

[Giuseppe Scrivano <gscrivan@xxxxxxxxxx>]

the reason is to avoid a delay caused by the synchronize_rcu() call in
kern_umount() when the mqueue mount is freed.

the code:

\#define _GNU_SOURCE
\#include <sched.h>
\#include <error.h>
\#include <errno.h>
\#include <stdlib.h>
int main()
{
  int i;
  for (i  = 0; i < 1000; i++)
    if (unshare (CLONE_NEWIPC) < 0)
      error (EXIT_FAILURE, errno, "unshare");
}

gets from:

	Command being timed: "./ipc-namespace"
	User time (seconds): 0.00
	System time (seconds): 0.06
	Percent of CPU this job got: 0%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:08.05

to:

	Command being timed: "./ipc-namespace"
	User time (seconds): 0.00
	System time (seconds): 0.02
	Percent of CPU this job got: 96%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.03

Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
---
v3:
- added comment for free_ipc_work
- commit message rephrased

v2: https://lkml.org/lkml/2020/2/17/839
- comment added in free_ipc_ns()

v1: https://lkml.org/lkml/2020/2/11/692

 include/linux/ipc_namespace.h |  2 ++
 ipc/namespace.c               | 24 ++++++++++++++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index c309f43bde45..a06a78c67f19 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -68,6 +68,8 @@ struct ipc_namespace {
 	struct user_namespace *user_ns;
 	struct ucounts *ucounts;
 
+	struct llist_node mnt_llist;
+
 	struct ns_common ns;
 } __randomize_layout;
 
diff --git a/ipc/namespace.c b/ipc/namespace.c
index b3ca1476ca51..c0470aef41a0 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -117,6 +117,10 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
 
 static void free_ipc_ns(struct ipc_namespace *ns)
 {
+	/* mq_put_mnt() waits for a grace period as kern_unmount()
+	 * uses synchronize_rcu().
+	 */
+	mq_put_mnt(ns);
 	sem_exit_ns(ns);
 	msg_exit_ns(ns);
 	shm_exit_ns(ns);
@@ -127,6 +131,21 @@ static void free_ipc_ns(struct ipc_namespace *ns)
 	kfree(ns);
 }
 
+static LLIST_HEAD(free_ipc_list);
+static void free_ipc(struct work_struct *unused)
+{
+	struct llist_node *node = llist_del_all(&free_ipc_list);
+	struct ipc_namespace *n, *t;
+
+	llist_for_each_entry_safe(n, t, node, mnt_llist)
+		free_ipc_ns(n);
+}
+
+/*
+ * The work queue is used to avoid the cost of synchronize_rcu in kern_unmount.
+ */
+static DECLARE_WORK(free_ipc_work, free_ipc);
+
 /*
  * put_ipc_ns - drop a reference to an ipc namespace.
  * @ns: the namespace to put
@@ -148,8 +167,9 @@ void put_ipc_ns(struct ipc_namespace *ns)
 	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
 		mq_clear_sbinfo(ns);
 		spin_unlock(&mq_lock);
-		mq_put_mnt(ns);
-		free_ipc_ns(ns);
+
+		if (llist_add(&ns->mnt_llist, &free_ipc_list))
+			schedule_work(&free_ipc_work);
 	}
 }
 
-- 
2.24.1