Re: [PATCH v6] taskstats: fix data-race

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

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