linux-kernel - Re: [PATCH] rcu: Remove unreachable logic

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Date: Mon, 29 Apr 2024 13:43:10 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Alan Huang <mmpgouride@...il.com>, paulmck@...nel.org,
 frederic@...nel.org, neeraj.upadhyay@...nel.org, joel@...lfernandes.org,
 josh@...htriplett.org, boqun.feng@...il.com, rostedt@...dmis.org,
 jiangshanlai@...il.com, qiang.zhang1211@...il.com
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Remove unreachable logic

On 2024-04-29 13:34, Alan Huang wrote:
> call_rcu_core is only called from __call_rcu_common with interrupt
> disabled. This patch thus removes the unreachable logic and the would-be
> unused 'flags' parameter.

Nack.

call_rcu_core() receives a @flags parameter which are the _saved_ flags
as they were prior to local_irq_save().

This patch highlights a misunderstanding of what the code is actually
doing.

Thanks,

Mathieu

> 
> Signed-off-by: Alan Huang <mmpgouride@...il.com>
> ---
>   kernel/rcu/tree.c | 35 ++---------------------------------
>   1 file changed, 2 insertions(+), 33 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d5507ac1bbf1..b0ea2ebd7769 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2983,7 +2983,7 @@ static void rcutree_enqueue(struct rcu_data *rdp, struct rcu_head *head, rcu_cal
>    * Handle any core-RCU processing required by a call_rcu() invocation.
>    */
>   static void call_rcu_core(struct rcu_data *rdp, struct rcu_head *head,
> -			  rcu_callback_t func, unsigned long flags)
> +			  rcu_callback_t func)
>   {
>   	rcutree_enqueue(rdp, head, func);
>   	/*
> @@ -2992,37 +2992,6 @@ static void call_rcu_core(struct rcu_data *rdp, struct rcu_head *head,
>   	 */
>   	if (!rcu_is_watching())
>   		invoke_rcu_core();
> -
> -	/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
> -	if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
> -		return;
> -
> -	/*
> -	 * Force the grace period if too many callbacks or too long waiting.
> -	 * Enforce hysteresis, and don't invoke rcu_force_quiescent_state()
> -	 * if some other CPU has recently done so.  Also, don't bother
> -	 * invoking rcu_force_quiescent_state() if the newly enqueued callback
> -	 * is the only one waiting for a grace period to complete.
> -	 */
> -	if (unlikely(rcu_segcblist_n_cbs(&rdp->cblist) >
> -		     rdp->qlen_last_fqs_check + qhimark)) {
> -
> -		/* Are we ignoring a completed grace period? */
> -		note_gp_changes(rdp);
> -
> -		/* Start a new grace period if one not already started. */
> -		if (!rcu_gp_in_progress()) {
> -			rcu_accelerate_cbs_unlocked(rdp->mynode, rdp);
> -		} else {
> -			/* Give the grace period a kick. */
> -			rdp->blimit = DEFAULT_MAX_RCU_BLIMIT;
> -			if (READ_ONCE(rcu_state.n_force_qs) == rdp->n_force_qs_snap &&
> -			    rcu_segcblist_first_pend_cb(&rdp->cblist) != head)
> -				rcu_force_quiescent_state();
> -			rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
> -			rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
> -		}
> -	}
>   }
>   
>   /*
> @@ -3121,7 +3090,7 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
>   	if (unlikely(rcu_rdp_is_offloaded(rdp)))
>   		call_rcu_nocb(rdp, head, func, flags, lazy);
>   	else
> -		call_rcu_core(rdp, head, func, flags);
> +		call_rcu_core(rdp, head, func);
>   	local_irq_restore(flags);
>   }
>   

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com