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Subject[PATCH] ipc: use a work queue to free_ipc
Date
it avoids blocking on synchronize_rcu() in kern_umount().

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@redhat.com>
---
include/linux/ipc_namespace.h | 2 ++
ipc/namespace.c | 17 +++++++++++++++--
2 files changed, 17 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..37d27e1b807a 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -117,6 +117,7 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,

static void free_ipc_ns(struct ipc_namespace *ns)
{
+ mq_put_mnt(ns);
sem_exit_ns(ns);
msg_exit_ns(ns);
shm_exit_ns(ns);
@@ -127,6 +128,17 @@ 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);
+}
+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 +160,9 @@ 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
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