[PATCH 5/5] arm: Enable IRQs before attempting to read user space in __und_usr

[Russell King - ARM Linux]

On Thu, Nov 21, 2013 at 09:35:34AM +0000, Alexey Ignatov wrote:
> Catalin Marinas <catalin.marinas <at> arm.com> writes:
> 
> > The Undef abort handler in the kernel reads the undefined instruction
> > from user space. If the page table was modified from another CPU, the
> > user access could fail and do_page_fault() will be executed with
> > interrupts disabled. This can potentially deadlock on ARM11MPCore or on
> > Cortex-A15 with erratum 798181 workaround enabled (both implying IPI for
> > TLB maintenance with page table lock held).
> > 
> > This patch enables the IRQs in __und_usr before attempting to read the
> > instruction from user space.
> 
> This patch moves enable_irq call from do_fpe directly to __und_usr handler,
> but __und_svc handler also calls do_fpe (via call_fpe), so now this codepath
> runs with disabled irqs. This behavior change doesn't look good for me.

However, you're not executing FPA instructions in the kernel as a general
rule, so it doesn't matter.