linux-kernel - Re: [PATCH v2 -rcu] srcu: Use rcu_seq_done

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Z8h_3xQWGNjIw7Uh@tardis>
Date: Wed, 5 Mar 2025 08:46:23 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>, rcu@...r.kernel.org
Subject: Re: [PATCH v2 -rcu] srcu: Use rcu_seq_done_exact() for polling API

On Wed, Feb 19, 2025 at 07:43:08AM -0500, Joel Fernandes wrote:
> poll_state_synchronize_srcu() uses rcu_seq_done() unlike
> poll_state_synchronize_rcu() which uses rcu_seq_done_exact().
> 
> The  rcu_seq_done_exact() makes more sense for polling API, as with
> this API, there is a higher chance that there is a significant delay
> between the get_state..() and poll_state..() calls since a cookie
> can be stored and reused at a later time. During such a delay, if
> the gp_seq counter progresses more than ULONG_MAX/2 distance, then
> poll_state..() may return false for a long time unwantedly.
> 
> Fix by using the more accurate rcu_seq_done_exact() API which is
> exactly what straight RCU's polling does.
> 
> It may make sense, as future work, to add debug code here as well, where
> we compare a physical timestamp between get_state..() and poll_state()
> calls and yell if significant time has past but the grace period has
> still not progressed.
> 
> Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>

Queued for further tests and reviews, thank you all.

Regards,
Boqun

> ---
> v1->v2: Resend with Neeraj review tag. Rebased on rcu/dev.
> 
>  kernel/rcu/srcutree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index d2a694944553..591371d62e89 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1589,7 +1589,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>  bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>  {
>  	if (cookie != SRCU_GET_STATE_COMPLETED &&
> -	    !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie))
> +	    !rcu_seq_done_exact(&ssp->srcu_sup->srcu_gp_seq, cookie))
>  		return false;
>  	// Ensure that the end of the SRCU grace period happens before
>  	// any subsequent code that the caller might execute.
> -- 
> 2.34.1
> 
>