From: Benjamin Herrenschmidt <benh@kernel•crashing.org>
To: Sebastien Dugue <sebastien.dugue@bull•net>
Cc: Tim Chavez <tinytim@us•ibm.com>,
Linux RT Users <linux-rt-users@vger•kernel.org>,
Jean Pierre Dion <jean-pierre.dion@bull•net>,
linux-kernel <linux-kernel@vger•kernel.org>,
"linuxppc-dev@ozlabs•org" <linuxppc-dev@ozlabs•org>,
paulus@samba•org, Gilles Carry <Gilles.Carry@ext•bull.net>
Subject: Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree
Date: Thu, 24 Jul 2008 08:17:38 +1000 [thread overview]
Message-ID: <1216851458.11027.346.camel@pasglop> (raw)
In-Reply-To: <20080723165932.57e3ada2@bull.net>
> The root cause of this bug lies in the fact that the XICS interrupt controller
> uses a radix tree for its reverse irq mapping and that we cannot allocate the tree
> nodes (even GFP_ATOMIC) with preemption disabled.
Is that yet another caes of -rt changing some basic kernel semantics ?
> In fact, we have 2 nested preemption disabling when we want to allocate
> a new node:
>
> - setup_irq() does a spin_lock_irqsave() before calling xics_startup() which
> then calls irq_radix_revmap() to insert a new node in the tree
>
> - irq_radix_revmap() also does a spin_lock_irqsave() (in irq_radix_wrlock())
> before the radix_tree_insert()
>
> The first patch moves the call to irq_radix_revmap() from xics_startup() out to
> xics_host_map_direct() and xics_host_map_lpar() which are called with preemption
> enabled.
I suppose that would work.
> The second patch is a little more involved in that it takes advantage of
> the concurrent radix tree to simplify the locking requirements and allows
> to allocate a new node outside a preemption disabled section.
>
> I just hope I've correctly understood the concurrent radix trees semantic
> and got the (absence of) locking right.
Hrm, that will need some scrutinity.
Thanks for looking at this.
Cheers,
Ben.
next prev parent reply other threads:[~2008-07-23 22:18 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-07-23 14:59 [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree Sebastien Dugue
2008-07-23 15:00 ` [PATCH 1/2][RT] powerpc - XICS: move the call to irq_radix_revmap from xics_startup to xics_host_map Sebastien Dugue
2008-07-23 15:01 ` [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix tree lockless Sebastien Dugue
2008-07-23 22:17 ` Benjamin Herrenschmidt [this message]
2008-07-24 11:08 ` [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree Sebastien Dugue
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