linux-kernel - Re: [PATCH] once: Don't use a work queue to reset sleepable static key

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Message-ID: <c27ff6a4-505c-4873-9e41-d52a21a1eba8@intel.com>
Date: Thu, 8 Jan 2026 16:33:18 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Andrew Morton <akpm@...ux-foundation.org>
CC: Eric Dumazet <edumazet@...gle.com>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH] once: Don't use a work queue to reset sleepable static
 key

Hi Tony,

On 1/8/26 3:27 PM, Tony Luck wrote:
> Pointless overhead to use a work queue to reset the static key
> for a DO_ONCE_SLEEPABLE() invocation.
> 
> Reported-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  lib/once.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/once.c b/lib/once.c
> index 2c306f0e891e..d83bdde78ced 100644
> --- a/lib/once.c
> +++ b/lib/once.c
> @@ -93,6 +93,7 @@ void __do_once_sleepable_done(bool *done, struct static_key_true *once_key,
>  {
>  	*done = true;
>  	mutex_unlock(&once_mutex);
> -	once_disable_jump(once_key, mod);
> +	BUG_ON(!static_key_enabled(once_key));
> +	static_branch_disable(once_key);

Can this be simplified more by disabling once_key with once_mutex held to eliminate the need for the
done boolean? Unless static_branch_disable() is considered to be so slow that it should also be
avoided in sleepable context, deferring to a mutex protected boolean until it completes ...
If "done" boolean is removed this may need an additional check of once_key within
__do_once_sleepable_start() just in case a second caller was blocked on once_mutex.

Reinette