Re: [PATCH 2/2] posix-timers: Use RCU in posix_timer_add()

[David Laight]

On Fri, 14 Feb 2025 13:59:11 +0000
Eric Dumazet <edumazet@xxxxxxxxxx> wrote:

> If many posix timers are hashed in posix_timers_hashtable,
> hash_lock can be held for long durations.
> 
> This can be really bad in some cases as Thomas
> explained in https://lore.kernel.org/all/87ednpyyeo.ffs@tglx/
> 
> We can perform all searches under RCU, then acquire
> the lock only when there is a good chance to need it,
> and after cpu caches were populated.
> 
> I also added a cond_resched() in the possible long loop.
> 
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> ---
>  kernel/time/posix-timers.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> index 204a351a2fd3..dd2f9016d3dc 100644
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -112,7 +112,19 @@ static int posix_timer_add(struct k_itimer *timer)
>  
>  		head = &posix_timers_hashtable[hash(sig, id)];
>  
> +		rcu_read_lock();
> +		if (__posix_timers_find(head, sig, id)) {
> +			rcu_read_unlock();
> +			cond_resched();
> +			continue;
> +		}
> +		rcu_read_unlock();
>  		spin_lock(&hash_lock);
> +		/*
> +		 * We must perform the lookup under hash_lock protection
> +		 * because another thread could have used the same id.
> +		 * This is very unlikely, but possible.
> +		 */

If next_posix_timer_id is 64bit (so can't wrap) I think you can compare the
(unmasked by MAX_INT) value being used with the current value.
If the difference is small (well less than MAX_INT) I don't think you need
the rescan.
(Not going to help 32bit - but who cares :-)

	David

>  		if (!__posix_timers_find(head, sig, id)) {
>  			hlist_add_head_rcu(&timer->t_hash, head);
>  			spin_unlock(&hash_lock);