linux-kernel - Re: [PATCH v7 1/6] lib/dlock-list: Distributed and lock-protected lists

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Date:   Fri, 13 Oct 2017 17:10:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v7 1/6] lib/dlock-list: Distributed and lock-protected
 lists

On 10/10/2017 01:35 AM, Boqun Feng wrote:
> On Thu, Oct 05, 2017 at 06:43:23PM +0000, Waiman Long wrote:
> [...]
>> +/*
>> + * As all the locks in the dlock list are dynamically allocated, they need
>> + * to belong to their own special lock class to avoid warning and stack
>> + * trace in kernel log when lockdep is enabled. Statically allocated locks
>> + * don't have this problem.
>> + */
>> +static struct lock_class_key dlock_list_key;
>> +
> So in this way, you make all dlock_lists share the same lock_class_key,
> which means if there are two structures:
>
> 	struct some_a {
> 		...
> 		struct dlock_list_heads dlists;
> 	};
>
> 	struct some_b {
> 		...
> 		struct dlock_list_heads dlists;
> 	};
>
> some_a::dlists and some_b::dlists are going to have the same lockdep
> key, is this what you want? If not, you may want to do something like
> init_srcu_struct() does.

I think it will be a problem only if a task acquire a lock in a
dlock-list and then acquire another lock from another dlock-list. The
way the dlock-list is used, no more than one lock will be acquired at
any time, so there won't be nested locking within the same dlock-list.
It is not a problem with the current use cases that I and Davidlohr
have, but it may be a problem if dlock-list becomes more widely used. I
will take a look at how init_srcu_struct() does, and maybe update the
patch accordingly. Thanks for the suggestion.

>
>> + * dlock_lists_del - Delete a node from a dlock list
>> + * @node : The node to be deleted
>> + *
>> + * We need to check the lock pointer again after taking the lock to guard
>> + * against concurrent deletion of the same node. If the lock pointer changes
>> + * (becomes NULL or to a different one), we assume that the deletion was done
>> + * elsewhere. A warning will be printed if this happens as it is likely to be
>> + * a bug.
>> + */
>> +void dlock_lists_del(struct dlock_list_node *node)
>> +{
>> +	struct dlock_list_head *head;
>> +	bool retry;
>> +
>> +	do {
>> +		head = READ_ONCE(node->head);
> Since we read node->head locklessly here, I think we should use
> WRITE_ONCE() for all the stores of node->head, to avoid store tearings?

Yes, you are right. I will use WRITE_ONCE() in my next version.

Cheers,
Longman