Re: [PATCH 2/9] ARM: change NR_IPIS to 8

[Arnd Bergmann]

On Thursday 18 February 2016 14:37:09 Russell King - ARM Linux wrote:
> On Thu, Feb 18, 2016 at 03:01:54PM +0100, Arnd Bergmann wrote:
> > When function tracing for IPIs is enabled, we get a warning for an
> > overflow of the ipi_types array with the IPI_CPU_BACKTRACE type
> > as triggered by raise_nmi():
> > 
> > arch/arm/kernel/smp.c: In function 'raise_nmi':
> > arch/arm/kernel/smp.c:489:2: error: array subscript is above array bounds [-Werror=array-bounds]
> >   trace_ipi_raise(target, ipi_types[ipinr]);
> 
> We really don't want to treat the backtrace IPI as a normal IPI at all -
> we want it to invoke the least amount of code possible.  Hence this code
> which avoids the issue:
> 
>         if ((unsigned)ipinr < NR_IPI) {
>                 trace_ipi_entry_rcuidle(ipi_types[ipinr]);
>                 __inc_irq_stat(cpu, ipi_irqs[ipinr]);
>         }
> 
> However, what's missing is that the addition of tracing here missed
> that CPU_BACKTRACE is not to be traced.  The call in raise_nmi()
> should have been converted to __smp_cross_call() to avoid the
> tracing code.

I've replaced the patch locally with the version below now, and
will throw it into the randconfig build test infrastructure to
make sure I didn't screw up in an obvious way here.

	Arnd