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SubjectRe: sched: delayed cleanup of user_struct
On Mon, Mar 09, 2009 at 07:37:17PM +0100, Kay Sievers wrote:
> From: Kay Sievers <kay.sievers@vrfy.org>
> Subject: sched: delayed cleanup of user_struct
>
> During bootup performance tracing we see repeated occurrences of
> /sys/kernel/uid/* events for the same uid, leading to a,
> in this case, rather pointless userspace processing for the
> same uid over and over.
>
> This is usally caused by tools which change their uid to "nobody",
> to run without privileges to read data supplied by untrusted users.
>
> This change delays the execution of the (already existing) scheduled
> work, to cleanup the uid after 0.5 seconds, so the allocated and announced
> uid can possibly be re-used by another process.
>
> This is the current behavior, where almost every invocation of a
> binary, which changes the uid, creates two events:
> $ read START < /sys/kernel/uevent_seqnum; \
> for i in `seq 100`; do su --shell=/bin/true bin; done; \
> read END < /sys/kernel/uevent_seqnum; \
> echo $(($END - $START))
> 178
>
> With the delayed cleanup, we get only two events, and userspace finishes
> a bit faster too:
> $ read START < /sys/kernel/uevent_seqnum; \
> for i in `seq 100`; do su --shell=/bin/true bin; done; \
> read END < /sys/kernel/uevent_seqnum; \
> echo $(($END - $START))
> 1
>

makes sense. I do have a patch though which changes some of the cleanup
code (fixing a memory leak) in -mm. These two patches will conflict.

> Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> Cc: Greg Kroah-Hartman <gregkh@suse.de>
> Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
> ---
> include/linux/sched.h | 2 +-
> kernel/user.c | 28 +++++++++++++---------------
> 2 files changed, 14 insertions(+), 16 deletions(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -670,7 +670,7 @@ struct user_struct {
> struct task_group *tg;
> #ifdef CONFIG_SYSFS
> struct kobject kobj;
> - struct work_struct work;
> + struct delayed_work work;
> #endif
> #endif
> };
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -75,6 +75,7 @@ static void uid_hash_remove(struct user_
> put_user_ns(up->user_ns);
> }
>
> +/* uidhash_lock must be held */
> static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
> {
> struct user_struct *user;
> @@ -82,7 +83,9 @@ static struct user_struct *uid_hash_find
>
> hlist_for_each_entry(user, h, hashent, uidhash_node) {
> if (user->uid == uid) {
> - atomic_inc(&user->__count);
> + /* possibly resurrect an "almost deleted" object */
> + if (atomic_inc_return(&user->__count) == 1)
> + cancel_delayed_work(&user->work);
> return user;
> }
> }
> @@ -283,12 +286,12 @@ int __init uids_sysfs_init(void)
> return uids_user_create(&root_user);
> }
>
> -/* work function to remove sysfs directory for a user and free up
> +/* delayed work function to remove the user and free up
> * corresponding structures.
> */
> -static void remove_user_sysfs_dir(struct work_struct *w)
> +static void remove_user_delayed(struct work_struct *w)
> {
> - struct user_struct *up = container_of(w, struct user_struct, work);
> + struct user_struct *up = container_of(w, struct user_struct, work.work);
> unsigned long flags;
> int remove_user = 0;
>
> @@ -299,15 +302,12 @@ static void remove_user_sysfs_dir(struct
> */
> uids_mutex_lock();
>
> - local_irq_save(flags);
> -
> - if (atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
> + spin_lock_irqsave(&uidhash_lock, flags);
> + if (atomic_read(&up->__count) == 0) {
> uid_hash_remove(up);
> remove_user = 1;
> - spin_unlock_irqrestore(&uidhash_lock, flags);
> - } else {
> - local_irq_restore(flags);
> }
> + spin_unlock_irqrestore(&uidhash_lock, flags);
>
> if (!remove_user)
> goto done;
> @@ -331,12 +331,8 @@ done:
> */
> static void free_user(struct user_struct *up, unsigned long flags)
> {
> - /* restore back the count */
> - atomic_inc(&up->__count);
> spin_unlock_irqrestore(&uidhash_lock, flags);
> -
> - INIT_WORK(&up->work, remove_user_sysfs_dir);
> - schedule_work(&up->work);
> + schedule_delayed_work(&up->work, msecs_to_jiffies(500));
> }
>
> #else /* CONFIG_USER_SCHED && CONFIG_SYSFS */
> @@ -442,6 +438,8 @@ struct user_struct *alloc_uid(struct use
> if (uids_user_create(new))
> goto out_destoy_sched;
>
> + INIT_DELAYED_WORK(&new->work, remove_user_delayed);
> +
> /*
> * Before adding this, check whether we raced
> * on adding the same user already..
>

--
regards,
Dhaval


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