linux-kernel - Re: [PATCH 2/9] ARM: change NR

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Message-ID: <20160218143709.GQ19428@n2100.arm.linux.org.uk>
Date:	Thu, 18 Feb 2016 14:37:09 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Nicolas Pitre <nico@...aro.org>,
	Jon Medhurst <tixy@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] ARM: change NR_IPIS to 8

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.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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