ipc/namespace.c: use a work queue to free_ipc

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"); } goes 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

Giuseppe Scrivano committed 6 years ago

Commit e1eb26f1 parent 4b78e202 files · +24 −2Browse files

Unified · Split

2 files

include/linux/ipc_namespace.h +2 −0
ipc/namespace.c +22 −2

include/linux/ipc_namespace.h+2−0

@@ -68,6 +68,8 @@ struct ipc_namespace { · 67 unmodified lines
68	68	struct user_namespace *user_ns;
69	69	struct ucounts *ucounts;
70	70
	71	+ struct llist_node mnt_llist;
	72	+
71	73	struct ns_common ns;
72	74	} __randomize_layout;
73	75

ipc/namespace.c+22−2

@@ -117,6 +117,10 @@ void free_ipcs(struct ipc_namespace ns, struct ipc_ids ids, · 116 unmodified lines
117	117
118	118	static void free_ipc_ns(struct ipc_namespace *ns)
119	119	{
	120	+ /* mq_put_mnt() waits for a grace period as kern_unmount()
	121	+ * uses synchronize_rcu().
	122	+ */
	123	+ mq_put_mnt(ns);
120	124	sem_exit_ns(ns);
121	125	msg_exit_ns(ns);
122	126	shm_exit_ns(ns);
@@ -127,6 +131,21 @@ static void free_ipc_ns(struct ipc_namespace *ns) · 4 unmodified lines
127	131	kfree(ns);
128	132	}
129	133
	134	+static LLIST_HEAD(free_ipc_list);
	135	+static void free_ipc(struct work_struct *unused)
	136	+{
	137	+ struct llist_node *node = llist_del_all(&free_ipc_list);
	138	+ struct ipc_namespace n, t;
	139	+
	140	+ llist_for_each_entry_safe(n, t, node, mnt_llist)
	141	+ free_ipc_ns(n);
	142	+}
	143	+
	144	+/*
	145	+ * The work queue is used to avoid the cost of synchronize_rcu in kern_unmount.
	146	+ */
	147	+static DECLARE_WORK(free_ipc_work, free_ipc);
	148	+
130	149	/*
131	150	* put_ipc_ns - drop a reference to an ipc namespace.
132	151	* @ns: the namespace to put
@@ -148,8 +167,9 @@ void put_ipc_ns(struct ipc_namespace *ns) · 15 unmodified lines
148	167	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
149	168	mq_clear_sbinfo(ns);
150	169	spin_unlock(&mq_lock);
151		− mq_put_mnt(ns);
152		− free_ipc_ns(ns);
	170	+
	171	+ if (llist_add(&ns->mnt_llist, &free_ipc_list))
	172	+ schedule_work(&free_ipc_work);
153	173	}
154	174	}
155	175