linux-kernel - Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Date:	Mon, 21 Mar 2016 10:49:18 +0100
From:	Jan Kara <jack@...e.cz>
To:	Waiman Long <Waiman.Long@....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>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

On Fri 18-03-16 15:44:01, Waiman Long wrote:
> +static __always_inline bool
> +__pcpu_list_next_cpu(struct pcpu_list_head *head, struct pcpu_list_state *state)
> +{
> +	if (state->lock)
> +		spin_unlock(state->lock);
> +next_cpu:
> +	/*
> +	 * for_each_possible_cpu(cpu)
> +	 */
> +	state->cpu = cpumask_next(state->cpu, cpu_possible_mask);
> +	if (state->cpu >= nr_cpu_ids)
> +		return false;	/* All the per-cpu lists iterated */
> +
> +	state->head = &per_cpu_ptr(head, state->cpu)->list;
> +	if (list_empty(state->head))
> +		goto next_cpu;
> +
> +	state->lock = &per_cpu_ptr(head, state->cpu)->lock;
> +	spin_lock(state->lock);
> +	state->curr = list_entry(state->head->next,
> +				 struct pcpu_list_node, list);
> +	return true;

Waiman, I repeat it for the third time as you keep ignoring it: This is
*racy*. The list for state->cpu can be empty by the time you acquire
state->lock and thus state->curr will point somewhere around the head of
the list but definitely not to a list member where it should.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR