linux-kernel - Re: [PATCH 1/2] list: introduce a new cutting helper

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Message-ID: <ZmsD_HDLBQAqOOU3@kbusch-mbp.dhcp.thefacebook.com>
Date: Thu, 13 Jun 2024 08:36:44 -0600
From: Keith Busch <kbusch@...nel.org>
To: Nilay Shroff <nilay@...ux.ibm.com>, l@...sch-mbp.dhcp.thefacebook.com
Cc: Keith Busch <kbusch@...a.com>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	hch@....de, sagi@...mberg.me, paulmck@...nel.org,
	davidgow@...gle.com, akpm@...ux-foundation.org,
	venkat88@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/2] list: introduce a new cutting helper

On Thu, Jun 13, 2024 at 07:11:52PM +0530, Nilay Shroff wrote:
> On 6/13/24 18:26, Keith Busch wrote:
> > But that's not the problem for the rcu case. It's the last line that's
> > the problem:
> > 
> >  	list->prev->next = list;
> > 
> > We can't change forward pointers for any element being detached from
> > @head because a reader iterating the list may see that new pointer value
> > and end up in the wrong list, breaking iteration. A synchronize rcu
> > needs to happen before forward pointers can be mucked with, so it still
> > needs to be done in two steps. Oh bother...
> 
> Agree and probably we may break it down using this API:
> static inline void list_cut_rcu(struct list_head *list,
>  		struct list_head *head, struct list_head *entry, 
> 		void (*sync)(void))
> {
>  	list->next = entry;
>  	list->prev = head->prev;
> 	__list_del(entry->prev, head);
> 	sync();
>  	entry->prev = list;
>  	list->prev->next = list;
> }

Yes, that's the pattern, but I think we need an _srcu() variant: the
"sync" callback needs to know the srcu_struct.