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

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 kernel error messages seen when trying to create UBI volume is

BUG: using smp_processor_id() in preemptible [00000000] code: ubi_bgt1d/843
caller is __udelay+0x14/0x70
Call Trace:
[<ffffffff8110b0d4>] dump_stack+0x8/0x34
[<ffffffff812ee1ac>] debug_smp_processor_id+0x114/0x130
[<ffffffff812e9274>] __udelay+0x14/0x70
[<ffffffff81337c0c>] cfi_amdstd_write_buffers+0xa9c/0xd70
[<ffffffff8134cab0>] ubi_io_sync_erase+0x248/0x390
[<ffffffff8134d714>] erase_worker+0x6c/0x4f8
[<ffffffff8134e4fc>] do_work+0xac/0x138
[<ffffffff8134e6a0>] ubi_thread+0x118/0x1a8
[<ffffffff8115ebe0>] kthread+0x88/0x90
[<ffffffff81115650>] kernel_thread_helper+0x10/0x18

---
 include/linux/mtd/cfi.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index 574d9ee..9673213 100644
@@ -495,7 +495,9 @@ static inline void cfi_udelay(int us)
 	if (us >= 1000) {
 		msleep((us+999)/1000);
 	} else {
+		preempt_disable();
 		udelay(us);
+		preempt_enable();
 		cond_resched();
 	}
 }

1.6.3.3.333.g4d53f

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

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(), or using raw_smp_processor_id() in it, after reading the above git commit's message. A proper fix would accept a context switch during the delay and rescale the remaining count, but even on x86 they haven't done that yet :-)

Regards,
-- Jamie

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.

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

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

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

This would definitely work.

Will look into this.

Thanks
Philby

It isn't really udelay() but smp_processor_id() that you are not to call from a preemptible thread. Now I also see Ed Swierk has done a similar thing https://patchwork.kernel.org/patch/4049/ and he comments "..which calls smp_processor_id(), which is not supposed to be called from a preemptible thread."

So perhaps I can use preempt_disable() around just this call in function __udelay()?

Regards,
Philby

Hello Jamie,

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