linux-kernel - Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks

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Date:	Wed, 17 Feb 2016 17:02:18 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Dave Chinner <david@...morbit.com>,
	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>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated
 per-cpu locks

On Wed, Feb 17, 2016 at 10:56:10AM -0500, Waiman Long wrote:
> >>+/**
> >>+ * for_all_percpu_list_entries - iterate over all the per-cpu list with locking
> >>+ * @pos:	the type * to use as a loop cursor for the current .
> >>+ * @next:	an internal type * variable pointing to the next entry
> >>+ * @pchead:	an internal struct list * of percpu list head
> >>+ * @pclock:	an internal variable for the current per-cpu spinlock
> >>+ * @head:	the head of the per-cpu list
> >>+ * @member:	the name of the per-cpu list within the struct
> >>+ */
> >>+#define for_all_percpu_list_entries(pos, next, pchead, pclock, head, member)\
> >>+	{								 \
> >>+	int cpu;							 \
> >>+	for_each_possible_cpu (cpu) {					 \
> >>+		typeof(*pos) *next;					 \
> >>+		spinlock_t *pclock = per_cpu_ptr(&(head)->lock, cpu);	 \
> >>+		struct list_head *pchead =&per_cpu_ptr(head, cpu)->list;\
> >>+		spin_lock(pclock);					 \
> >>+		list_for_each_entry_safe(pos, next, pchead, member.list)
> >>+
> >>+#define end_all_percpu_list_entries(pclock)	spin_unlock(pclock); } }

> I will try to shorten the name and better document the macro. This is
> probably the most tricky part of the whole part.

Note that your use of _safe() here actually makes the usage in
iterate_bdevs() unsafe!

Because that function relies on __iget() pinning the current position,
which means that once you re-acquire the list lock, pos->next is valid.

Howveer, _safe() takes pos->next before dropping the lock, and that
object is not actually pinned and can go away.