linux-kernel - Re: [PATCH] llist: Fix missing lockless

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Message-ID: <54DA0FC6.3090204@hurleysoftware.com>
Date:	Tue, 10 Feb 2015 09:03:50 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Pranith Kumar <bobby.prani@...il.com>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Huang Ying <ying.huang@...el.com>,
	linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] llist: Fix missing lockless_dereference()

On 02/06/2015 09:08 PM, Mathieu Desnoyers wrote:
> A lockless_dereference() appears to be missing in llist_del_first().
> It should only matter for Alpha in practice.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> CC: Huang Ying <ying.huang@...el.com>
> CC: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> CC: David Howells <dhowells@...hat.com>
> CC: Pranith Kumar <bobby.prani@...il.com>
> CC: stable@...r.kernel.org # v3.1+
> ---
>  lib/llist.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/llist.c b/lib/llist.c
> index f76196d..f34e176 100644
> --- a/lib/llist.c
> +++ b/lib/llist.c
> @@ -26,6 +26,7 @@
>  #include <linux/export.h>
>  #include <linux/interrupt.h>
>  #include <linux/llist.h>
> +#include <linux/rcupdate.h>

Pranith,

I didn't realize you put lockless_dereference() in rcupdate.h

If the point of lockless_reference() is to provide a utility function for
situations _not_ involving RCU, then it doesn't make sense to provide it
in an RCU header file.

Regards,
Peter Hurley

PS - That's not an objection to this patch, though.

>  /**
> @@ -67,7 +68,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
>  {
>  	struct llist_node *entry, *old_entry, *next;
>  
> -	entry = head->first;
> +	/*
> +	 * Load entry before entry->next. Matches the implicit
> +	 * memory barrier before the cmpxchg in llist_add_batch(),
> +	 * which ensures entry->next is stored before entry.
> +	 */
> +	entry = lockless_dereference(head->first);
>  	for (;;) {
>  		if (entry == NULL)
>  			return NULL;
> 

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