linux-kernel - RE: [PATCH] rcu/segcblist: Use WRITE

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Message-ID: <LV3PR11MB8768B531BD88260D26996285F509A@LV3PR11MB8768.namprd11.prod.outlook.com>
Date: Thu, 11 Sep 2025 15:46:08 +0000
From: "Kumar, Kaushlendra" <kaushlendra.kumar@...el.com>
To: "paulmck@...nel.org" <paulmck@...nel.org>
CC: "dave@...olabs.net" <dave@...olabs.net>, "josh@...htriplett.org"
	<josh@...htriplett.org>, "frederic@...nel.org" <frederic@...nel.org>,
	"neeraj.upadhyay@...nel.org" <neeraj.upadhyay@...nel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "rcu@...r.kernel.org" <rcu@...r.kernel.org>
Subject: RE: [PATCH] rcu/segcblist: Use WRITE_ONCE() for rclp->len decrement

On Thu, Sep 11, 2025 at 10:40:09AM +0530, Paul E. McKenney wrote:
> On Thu, Sep 11, 2025 at 10:40:09AM +0530, Kaushlendra Kumar wrote:
> > The rclp->len field is accessed concurrently by multiple contexts in 
> > RCU operations. Using WRITE_ONCE() provides the necessary memory 
> > ordering guarantees.
> 
> Could you please be specific here?  What calls to rcu_cblist_dequeue() are such that hte ->qlen field can be concurrently accessed?
> 
> (Full disclosure: I don't see any, and KCSAN hasn't found any.  Of course, that does not necessarily mean that there is no concurrent access.
> But we need such concurrent access called out explicitly here, because it might well be that the concurrent access is itself the bug.)
> 
> 							Thanx, Paul

Hi Paul,

Thank you for the clarification. You are absolutely correct. After reviewing the 
code more carefully, I cannot identify specific concurrent access patterns for 
the rclp->len field during rcu_cblist_dequeue() operations.

The primary motivation for this patch was to maintain consistency with 
rcu_cblist_enqueue(), which uses WRITE_ONCE() for the rclp->len increment. 

I will modify the message accordingly and send a patch.

Best regards,
Kaushlendra