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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 17 Feb 2016 12:26:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Waiman Long <Waiman.Long@....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 10:10:02PM +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.