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Date: Wed, 17 Feb 2016 12:36:18 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Waiman Long <Waiman.Long@hpe.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
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        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
Message-ID: <20160217113618.GO6375@twins.programming.kicks-ass.net>
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On Wed, Feb 17, 2016 at 12:26:54PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 17, 2016 at 10:10:02PM +1100, Dave Chinner wrote:
> > On Wed, Feb 17, 2016 at 12:00:40PM +0100, Peter Zijlstra wrote:
> 
> > > Yeah, that is pretty terrible. Maybe a visitor interface is advisable?
> > > 
> > > visit_percpu_list_entries(struct percpu_list *head, void (*visitor)(struct list_head *pos, void *data), void *data)
> > > {
> > > 	int cpu;
> > > 
> > > 	for_each_possible_cpu(cpu) {
> > > 		spinlock_t *lock = per_cpu_ptr(&head->lock, cpu);
> > > 		struct list_head *head = per_cpu_ptr(&head->list, cpu);
> > > 		struct list_head *pos, *tmp;
> > > 
> > > 		spin_lock(lock);
> > > 		for (pos = head->next, tmp = pos->next; pos != head; pos = tmp)
> > > 			visitor(pos, data);
> > 
> > I thought about this - it's the same problem as the list_lru walking
> > functions. That is, the visitor has to be able to drop the list lock
> > to do blocking operations, so the lock has to be passed to the
> > visitor/internal loop context somehow, and the way the callers can
> > use it need to be documented.
> 
> But you cannot drop the lock and guarantee fwd progress. The moment you
> drop the lock, you have to restart the iteration from the head, since
> any iterator you had might now be pointing into space.

Ah, I see what iterate_bdevs() does. Yes, that's somewhat 'special'. Not
sure it makes sense to craft a generic 'interface' for that.