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SubjectRe: [PATCH v6] taskstats: fix data-race
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On 21/10/2019 13.33, Christian Brauner wrote:
> The first approach used smp_load_acquire() and smp_store_release().
> However, after having discussed this it seems that the data dependency
> for kmem_cache_alloc() would be fixed by WRITE_ONCE().
> Furthermore, the smp_load_acquire() would only manage to order the stats
> check before the thread_group_empty() check. So it seems just using
> READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this
> up for discussion at least.
>
> /* v6 */
> - Christian Brauner <christian.brauner@ubuntu.com>:
> - bring up READ_ONCE()/WRITE_ONCE() approach for discussion
> ---
> kernel/taskstats.c | 26 +++++++++++++++-----------
> 1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> index 13a0f2e6ebc2..111bb4139aa2 100644
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -554,25 +554,29 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
> static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
> {
> struct signal_struct *sig = tsk->signal;
> - struct taskstats *stats;
> + struct taskstats *stats_new, *stats;
>
> - if (sig->stats || thread_group_empty(tsk))
> - goto ret;
> + /* Pairs with WRITE_ONCE() below. */
> + stats = READ_ONCE(sig->stats);
> + if (stats || thread_group_empty(tsk))
> + return stats;
>
> /* No problem if kmem_cache_zalloc() fails */
> - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
>
> spin_lock_irq(&tsk->sighand->siglock);
> - if (!sig->stats) {
> - sig->stats = stats;
> - stats = NULL;
> + if (!stats) {
> + stats = stats_new;
> + /* Pairs with READ_ONCE() above. */
> + WRITE_ONCE(sig->stats, stats_new);
> + stats_new = NULL;

No idea about the memory ordering issues, but don't you need to
load/check sig->stats again? Otherwise it seems that two threads might
both see !sig->stats, both allocate a stats_new, and both
unconditionally in turn assign their stats_new to sig->stats. Then the
first assignment ends up becoming a memory leak (and any writes through
that pointer done by the caller end up in /dev/null...)

Rasmus

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