linux-kernel - Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu

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Message-ID: <Z5QcXoE0lrS6yIBt@pavilion.home>
Date: Sat, 25 Jan 2025 00:03:58 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu_full() GP-start
 detection

Le Fri, Dec 13, 2024 at 11:49:49AM -0800, Paul E. McKenney a écrit :
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 2f9c9272cd486..d2a91f705a4ab 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -162,7 +162,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
>  {
>  	unsigned long cur_s = READ_ONCE(*sp);
>  
> -	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_STATE_MASK + 1));
> +	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1));

This might need a comment.

The way I understand it is that rcu_state.gp_seq might be seen started while
root_rnp->gp_seq is not. So rcu_seq_snap() on the started rcu_state.gp_seq
may return maximum 2 full GPs ahead of root_rnp->gp_seq. And therefore it takes below
2 GPs to safely deduce we wrapped around.

Should it be ULONG_CMP_LT(cur_s, s - (2 * (RCU_SEQ_STATE_MASK + 1))) ?

Or am I missing something?

Thanks.