[PATCH] arm64: mm: allow preemption in copy_to_user_page

[Catalin Marinas]

On Tue, Mar 22, 2016 at 10:11:28AM +0000, Mark Rutland wrote:
> Currently we disable preemption in copy_to_user_page; a behaviour that
> we inherited from the 32-bit arm code. This was necessary for older
> cores without broadcast data cache maintenance, and ensured that cache
> lines were dirtied and cleaned by the same CPU. On these systems dirty
> cache line migration was not possible, so this was sufficient to
> guarantee coherency.
> 
> On contemporary systems, cache coherence protocols permit (dirty) cache
> lines to migrate between CPUs as a result of speculation, prefetching,
> and other behaviours. To account for this, in ARMv8 data cache
> maintenance operations are broadcast and affect all data caches in the
> domain associated with the VA (i.e. ISH for kernel and user mappings).
> 
> In __switch_to we ensure that tasks can be safely migrated in the middle
> of a maintenance sequence, using a dsb(ish) to ensure prior explicit
> memory accesses are observed and cache maintenance operations are
> completed before a task can be run on another CPU.
> 
> Given the above, it is not necessary to disable preemption in
> copy_to_user_page. This patch removes the preempt_{disable,enable}
> calls, permitting preemption.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will.deacon at arm.com>

Applied. Thanks.

-- 
Catalin