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Subject: Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible
 ubi_bgt1d kthread
From:   Philby John <pjohn@mvista.com>
Reply-To: pjohn@mvista.com
To:     Jamie Lokier <jamie@shareable.org>
Cc:     linux-mtd@lists.infradead.org, linux-mips@linux-mips.org,
        David Daney <ddaney@caviumnetworks.com>,
        linux-kernel@vger.kernel.org,
        Artem Bityutskiy <dedekind1@gmail.com>
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Date:   Tue, 15 Jun 2010 17:56:34 +0530
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Hello Jamie,

On Mon, 2010-06-14 at 16:04 +0100, Jamie Lokier wrote:
> Philby John wrote:
> > mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
> > 
> > On a MIPS Cavium Octeon CN5020 when trying to create a UBI volume,
> > on the NOR flash, the kernel thread ubi_bgt1d calls
> > cfi_amdstd_write_buffers() --> do_write_buffer() -->
> > INVALIDATE_CACHE_UDELAY --> __udelay(). Its __udelay() that calls
> > smp_processor_id() in preemptible code, which you are not supposed to.
> > Fix the problem by disabling preemption.
> 
> The MTD code just calls udelay().
> Are you sure it isn't permitted to call udelay() from preemptible code?
> I think it is fine.
> 
> Perhaps MIPS udelay() should be disabling preemption itself, or
> (as x86 does) using raw_smp_processor_id() instead?

Sorry for the noise. I now find that raw_smp_processor_id() has been
implemented specific to MIPS in the latest kernel, I was using 2.6.32.

Thanks and regards,
Philby