Re: [PATCH] bsdacct: delete timer with sync intension

Andrew Morton
2010-03-31T19:57:21Z

On Thu, 25 Mar 2010 17:35:10 +0300
Vitaliy Gusev <vgusev@openvz.org> wrote:

> acct_exit_ns --> acct_file_reopen deletes timer without
> check timer execution on other CPUs. So acct_timeout() can
> change an unmapped memory.
>

That sounds ugly.

>
> ---
> kernel/acct.c | 17 +++++++++--------
> 1 files changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/acct.c b/kernel/acct.c
> index a6605ca..6ac80ca 100644
> --- a/kernel/acct.c
> +++ b/kernel/acct.c
> @@ -353,17 +353,18 @@ restart:
>
> void acct_exit_ns(struct pid_namespace *ns)
> {
> - struct bsd_acct_struct *acct;
> + struct bsd_acct_struct *acct = ns->bacct;
>
> - spin_lock(&acct_lock);
> - acct = ns->bacct;
> - if (acct != NULL) {
> - if (acct->file != NULL)
> - acct_file_reopen(acct, NULL, NULL);
> + if (acct == NULL)
> + return;
>
> - kfree(acct);
> - }
> + del_timer_sync(&acct->timer);
> + spin_lock(&acct_lock);
> + if (acct->file != NULL)
> + acct_file_reopen(acct, NULL, NULL);
> spin_unlock(&acct_lock);
> +
> + kfree(acct);
> }
>

Is this sufficient? acct_file_reopen() does a del_timer(), so
acct_timeout() could be running concurrently with acct_file_reopen(),
but acct_file_reopen() is merrily altering data at *acct.

Perhaps acct_file_reopen() should be using del_timer_sync()?

check_free_space() is doing a similar thing:

del_timer(&acct->timer);
acct->needcheck = 0;

the currently-running timer handler now goes and sets needcheck again!

Methinks the whole thing needs a bit of a rethink, bearing in mind how
del_timer() actually works.