Re: [PATCH 2/3] iommu/iova: don't disable preempt around this_cpu_ptr()

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

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.



	Joerg