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

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

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