* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
@ 2010-06-14 15:04 ` Jamie Lokier
0 siblings, 0 replies; 10+ messages in thread
From: Jamie Lokier @ 2010-06-14 15:04 UTC (permalink / raw)
To: Philby John
Cc: linux-mips, Artem Bityutskiy, linux-mtd, David Daney, linux-kernel
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? 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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
2010-06-14 15:04 ` Jamie Lokier
@ 2010-06-14 15:37 ` Philby John
-1 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-14 15:37 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mtd, linux-mips, David Daney, linux-kernel, Artem Bityutskiy
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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
@ 2010-06-14 15:37 ` Philby John
0 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-14 15:37 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mips, Artem Bityutskiy, linux-mtd, David Daney, linux-kernel
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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
2010-06-14 15:04 ` Jamie Lokier
@ 2010-06-14 16:40 ` Philby John
-1 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-14 16:40 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mtd, linux-mips, David Daney, linux-kernel, Artem Bityutskiy
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.
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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
@ 2010-06-14 16:40 ` Philby John
0 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-14 16:40 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mips, Artem Bityutskiy, linux-mtd, David Daney, linux-kernel
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.
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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
2010-06-14 15:04 ` Jamie Lokier
@ 2010-06-15 12:26 ` Philby John
-1 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-15 12:26 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mtd, linux-mips, David Daney, linux-kernel, Artem Bityutskiy
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
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] mtd: Fix bug using smp_processor_id() in preemptible ubi_bgt1d kthread
@ 2010-06-15 12:26 ` Philby John
0 siblings, 0 replies; 10+ messages in thread
From: Philby John @ 2010-06-15 12:26 UTC (permalink / raw)
To: Jamie Lokier
Cc: linux-mips, Artem Bityutskiy, linux-mtd, David Daney, linux-kernel
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
^ permalink raw reply [flat|nested] 10+ messages in thread