Re: [PATCH v7 1/3] llist: Add a lock-less list variant terminated by a sentinel node

From: Tejun Heo <tj@kernel.org>
To: Waiman Long <longman@redhat.com>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	cgroups@vger.kernel.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, "Ming Lei" <ming.lei@redhat.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Huang Ying" <ying.huang@intel.com>,
	"Michal Koutný" <mkoutny@suse.com>
Subject: Re: [PATCH v7 1/3] llist: Add a lock-less list variant terminated by a sentinel node
Date: Mon, 3 Oct 2022 06:40:04 -1000	[thread overview]
Message-ID: <YzsQZPONIJRgtf3o@slm.duckdns.org> (raw)
In-Reply-To: <20221003154459.207538-2-longman@redhat.com>

Hello, Waiman.

On Mon, Oct 03, 2022 at 11:44:57AM -0400, Waiman Long wrote:
> The lock-less list API is useful for dealing with list in a lock-less
> manner. However, one of the drawback of the current API is that there
> is not an easy way to determine if an entry has already been put into a
> lock-less list. This has to be tracked externally and the tracking will
> not be atomic unless some external synchronization logic is in place.
> 
> This patch introduces a new variant of the lock-less list terminated
> by a sentinel node instead of by NULL. The function names start with
> "sllist" instead of "llist". The advantage of this scheme is that we
> can atomically determine if an entry has been put into a lock-less
> list by looking at the next pointer of the llist_node. Of course, the
> callers must clear the next pointer when an entry is removed from the
> lockless list. This is done automatically when the sllist_for_each_safe
> or sllist_for_each_entry_safe iteraters are used. The non-safe versions
> of sllist iterator are not provided.

Any chance we can add sentinel to the existing llist instead of creating a
new variant? There's no real downside to always using sentinel, right?

Thanks.

-- 
tejun