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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.

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