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SubjectRe: [PATCH] llist: Fix missing lockless_dereference()
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Hi, Mathieu,

On Sun, 2015-02-08 at 04:25 +0000, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > From: "Michael Cree" <mcree@orcon.net.nz>
> > To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
> > Cc: "Greg KH" <gregkh@linuxfoundation.org>, linux-alpha@vger.kernel.org, "Richard Henderson" <rth@twiddle.net>, "Ivan
> > Kokshaysky" <ink@jurassic.park.msu.ru>, "Matt Turner" <mattst88@gmail.com>, "Huang Ying" <ying.huang@intel.com>,
> > linux-kernel@vger.kernel.org, "Paul McKenney" <paulmck@linux.vnet.ibm.com>, "David Howells" <dhowells@redhat.com>,
> > "Pranith Kumar" <bobby.prani@gmail.com>, stable@vger.kernel.org
> > Sent: Saturday, February 7, 2015 7:47:29 PM
> > Subject: Re: [PATCH] llist: Fix missing lockless_dereference()
> >
> > On Sat, Feb 07, 2015 at 10:30:44PM +0000, Mathieu Desnoyers wrote:
> > > > On Fri, Feb 06, 2015 at 09:08:21PM -0500, Mathieu Desnoyers wrote:
> > > > > A lockless_dereference() appears to be missing in llist_del_first().
> > > > > It should only matter for Alpha in practice.
> >
> > What could one anticipate to be the symptoms of such a missing
> > lockless_dereference()?
>
> This can trigger corruption of the lockless linked-list, which is
> used across a few subsystems. AFAIU, the scenario is as follows.
> Please bear with me, because it's been a while since I've read on
> the Alpha multi-cache-banks behavior.
>
> The list here would be initially non-empty. Initial state of
> new_last->next is unset (newly allocated); IOW: garbage. CPU A
> adds a node into the list while CPU B removes a node from the
> head of the list.
>
> CPU A CPU B
> llist_add_batch()
> - Stores to new_last->next
> - implicit full mb before cmpxchg makes the
> update to CPU A's cache bank containing
> new_last->next visible to other CPUs
> before CPU A's cache bank update making
> head->first visible to other CPUs.
> - cmpxchg updates head->first = new_first
> llist_del_first()
> - entry = load head->first
> -> here, lack of barrier on Alpha creates a window where
> CPU B's cache bank can see the updated "head->first",
> but the cache bank holding the next value did not
> receive the update yet, since each cache bank have
> their own channel, which can be independently
> saturated.
> - next = load entry->next (dereference entry pointer)
> - cmpxchg updates head->first = next
> -> can store unset "next" value into head->first, thus
> corrupting the linked list.

If my understanding were correct, cmpxchg will imply a full mb before
and after it, so that there is a mb between load head->first in cmpxchg
and load entry->next. If so, the memory barrier is only needed before
the loop.

Best Regards,
Huang, Ying

> >
> > The Alpha kernel is behaving pretty well provided one builds a machine
> > specific kernel and UP. When running an SMP kernel some packages
> > (most notably the java runtime, but there are a few others) occasionally
> > lock up in a pthread call --- could be a problem in libc rather then the
> > kernel.
>
> Are those lockups always occasional, or you have ways to reproduce them
> frequently with stress-tests ?
>
> Thanks,
>
> Mathieu
>
> >
> > > > Meta-comment, do we really care about Alpha anymore? Is it still
> > > > consered an "active" arch we support?
> >
> > There are a few of us still running recent kernels on Alpha. I am
> > maintaining the unofficial Debian alpha port at debian-ports, and the
> > Debian popcon shows about 10 installations of Debian Alpha.
> >
> > Cheers
> > Michael.
> >
>




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