[PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread

[Philby John]

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.


The mips code uses __udelay() where the macro current_cpu_data returns
the actual data structure on a per CPU basis by calling
smp_processor_id(). Since I have enabled CONFIG_DEBUG_PREEMPT, this
would call debug_smp_processor_id(). This function would check

a)if the thread is preemptiable. If preemption is disabled, normal flow.
b)If irqs are disabled, if yes normal flow.
c)if the thread is bound to a single cpu, if yes normal flow
d)or if its an early bootup

None of these condition get satisfied and hence the kernel error
messages are seen. So I think yes for MIPS, udelay() shouldn't be called
in preemptiable code.

> 
> Perhaps MIPS udelay() should be disabling preemption itself,

I will need to investigate this. Will follow up soon.

>  or
> (as x86 does) using raw_smp_processor_id() instead?

I have enabled CONFIG_DEBUG_PREEMPT so this would call
debug_smp_processor_id() instead of raw_smp_processor_id().

>   Or perhaps the x86
> version is a bug because the current CPU might change during the delay loop?
> 

Yes, isn't this a possibility? In that case shouldn't we be using
spin_lock_irqsave() ?

> See git commit 5c1ea08215f1f830dfaf4819a5f22efca41c3832
> "x86: enable preemption in delay"
> 
> I don't think it makes sense to disable preemption in all udelay()
> calls in drivers, so my NAK to this MTD patch.  To workaround,
> consider putting the preempt_disable in MIPS udelay(),

This would definitely work.

>  or using
> raw_smp_processor_id() in it, after reading the above git commit's
> message.

Will look into this.

Thanks
Philby