| Message ID | 1276513457.16642.3.camel@localhost.localdomain |
|---|---|
| State | New, archived |
| Headers | show |
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
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
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
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
diff --git a/include/linux/mtd/cfi.h b/include/linux/mtd/cfi.h index 574d9ee..9673213 100644 --- a/include/linux/mtd/cfi.h +++ b/include/linux/mtd/cfi.h @@ -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(); } }
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 Signed-off-by: Philby John <pjohn@mvista.com> --- include/linux/mtd/cfi.h | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-)