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

From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Dave Chinner <david@fromorbit.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Kara <jack@suse.com>, Jeff Layton <jlayton@poochiereds.net>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>, Andi Kleen <andi@firstfloor.org>,
	Dave Chinner <dchinner@redhat.com>,
	Scott J Norton <scott.norton@hp.com>,
	Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks
Date: Wed, 17 Feb 2016 17:02:18 +0100	[thread overview]
Message-ID: <20160217160218.GG6357@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <56C4981A.8040705@hpe.com>

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.