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
