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

Signed-off-by: Giuseppe Scrivano <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Reviewed-by: Waiman Long <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Manfred Spraul <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>
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giuseppe authored and torvalds committed Jun 8, 2020
1 parent 4b78e20 commit e1eb26f
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Showing 2 changed files with 24 additions and 2 deletions.
2 changes: 2 additions & 0 deletions include/linux/ipc_namespace.h
Original file line number Diff line number Diff line change
Expand Up @@ -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;

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24 changes: 22 additions & 2 deletions ipc/namespace.c
Original file line number Diff line number Diff line change
Expand Up @@ -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);
Expand All @@ -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
Expand All @@ -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);
}
}

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