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Re: BUG: using smp_processor_id() in preemptible code

To: Pavel Kiryukhin <vksavl@gmail.com>
Subject: Re: BUG: using smp_processor_id() in preemptible code
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 28 Nov 2007 11:23:39 +0000
Cc: linux-mips@linux-mips.org
In-reply-to: <73cd086a0711270820r92dc164heaa453e94cf1a8c6@mail.gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <73cd086a0711270820r92dc164heaa453e94cf1a8c6@mail.gmail.com>
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On Tue, Nov 27, 2007 at 07:20:47PM +0300, Pavel Kiryukhin wrote:

> I get the following bug running 2.6.18 on malta34Kc .
> 
> Freeing prom memory: 956kb freed
> Freeing firmware memory: 978944k freed
> Freeing unused kernel memory: 180k freed
> BUG: using smp_processor_id() in preemptible [00000000] code: swapper/1
> caller is r4k_dma_cache_wback_inv+0x144/0x2a0
> Call Trace:
>  [<80117af8>] r4k_dma_cache_wback_inv+0x144/0x2a0
>  [<802e4b84>] debug_smp_processor_id+0xd4/0xf0
>  [<802e4b7c>] debug_smp_processor_id+0xcc/0xf0
> ...
> CONFIG_DEBUG_PREEMPT is enabled.
> --
> Bug cause is blast_dcache_range() in preemptible code [in
> r4k_dma_cache_wback_inv()].
> blast_dcache_range() is constructed via __BUILD_BLAST_CACHE_RANGE that
> uses cpu_dcache_line_size(). It uses current_cpu_data that use
> smp_processor_id() in turn. In case of CONFIG_DEBUG_PREEMPT
> smp_processor_id emits BUG if we are executing with preemption
> enabled.
> 
> Cpu options of cpu0 are assumed to be the superset of all processors.
> 
> Can I make the same assumptions for cache line size  and fix this
> issue the following way:

That's safe and probably saner than littlering preempt_disable /
preempt_enable calls over the code.

  Ralf

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