linux-kernel - Re: [PATCH 2/3] iommu/iova: don't disable preempt around this_cpu

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Date:   Wed, 28 Jun 2017 11:31:55 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] iommu/iova: don't disable preempt around
 this_cpu_ptr()

On 2017-06-28 11:22:05 [+0200], Joerg Roedel wrote:
> On Tue, Jun 27, 2017 at 06:16:47PM +0200, Sebastian Andrzej Siewior wrote:
> > Commit 583248e6620a ("iommu/iova: Disable preemption around use of
> > this_cpu_ptr()") disables preemption while accessing a per-CPU variable.
> > This does keep lockdep quiet. However I don't see the point why it is
> > bad if we get migrated after its access to another CPU.
> > __iova_rcache_insert() and __iova_rcache_get() immediately locks the
> > variable after obtaining it - before accessing its members.
> > _If_ we get migrated away after retrieving the address of cpu_rcache
> > before taking the lock then the *other* task on the same CPU will
> > retrieve the same address of cpu_rcache and will spin on the lock.
> > 
> > alloc_iova_fast() disables preemption while invoking
> > free_cpu_cached_iovas() on each CPU. The function itself uses
> > per_cpu_ptr() which does not trigger a warning (like this_cpu_ptr()
> > does). It _could_ make sense to use get_online_cpus() instead but the we
> > have a hotplug notifier for CPU down (and none for up) so we are good.
> 
> Does that really matter? The spin_lock disables irqs and thus avoids
> preemption too. We also can't get rid of the irqsave lock here because
> these locks are taken in the dma-api path which is used from interrupt
> context.

It really does. The spin_lock() does disable preemption but this is not
the problem. The thing is that the preempt_disable() is superfluous and
it hurts Preempt-RT (and this is how I noticed it). Also the
get_cpu_ptr() is not requited and was only added to keep lockdep quiet
(according to the history).
Everything else here can stay as-is, I am just asking for the removal of
the redundant preempt_disable() where it is not required.

> 	Joerg

Sebastian