Messages in this thread | | | Date | Wed, 28 Jun 2017 11:31:55 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH 2/3] iommu/iova: don't disable preempt around this_cpu_ptr() |
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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
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