Re: [PATCH] llist: Fix missing lockless_dereference()

[Huang Ying]

Hi, Mathieu,

On Sun, 2015-02-08 at 04:25 +0000, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > From: "Michael Cree" <mcree@xxxxxxxxxxxx>
> > To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: "Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx>, linux-alpha@xxxxxxxxxxxxxxx, "Richard Henderson" <rth@xxxxxxxxxxx>, "Ivan
> > Kokshaysky" <ink@xxxxxxxxxxxxxxxxxxxx>, "Matt Turner" <mattst88@xxxxxxxxx>, "Huang Ying" <ying.huang@xxxxxxxxx>,
> > linux-kernel@xxxxxxxxxxxxxxx, "Paul McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, "David Howells" <dhowells@xxxxxxxxxx>,
> > "Pranith Kumar" <bobby.prani@xxxxxxxxx>, stable@xxxxxxxxxxxxxxx
> > 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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