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Re: [PATCH] MIPS: Protect current_cpu_data with preempt disable in delay

To: Yang Shi <yang.shi@windriver.com>
Subject: Re: [PATCH] MIPS: Protect current_cpu_data with preempt disable in delay()
From: Yang Shi <yang.shi@windriver.com>
Date: Mon, 08 Mar 2010 17:42:34 +0800
Cc: David Daney <ddaney@caviumnetworks.com>, ralf@linux-mips.org, linux-mips@linux-mips.org
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Hi folks,

Any advice on this topic?

Thanks,
Yang

Yang Shi 写道:
David Daney 写道:
On 03/04/2010 01:39 AM, Yang Shi wrote:
During machine restart with reboot command, get the following
bug info:

BUG: using smp_processor_id() in preemptible [00000000] code: reboot/1989
caller is __udelay+0x14/0x70
Call Trace:
[<ffffffff8110ad28>] dump_stack+0x8/0x34
[<ffffffff812dde04>] debug_smp_processor_id+0xf4/0x110
[<ffffffff812d90bc>] __udelay+0x14/0x70
[<ffffffff81378274>] md_notify_reboot+0x12c/0x148
[<ffffffff81161054>] notifier_call_chain+0x64/0xc8
[<ffffffff811614dc>] __blocking_notifier_call_chain+0x64/0xc0
[<ffffffff8115566c>] kernel_restart_prepare+0x1c/0x38
[<ffffffff811556cc>] kernel_restart+0x14/0x50
[<ffffffff8115581c>] SyS_reboot+0x10c/0x1f0
[<ffffffff81103684>] handle_sysn32+0x44/0x84

The root cause is that current_cpu_data is accessed in preemptible
context, so protect it with preempt_disable/preempt_enable pair
in delay().

Signed-off-by: Yang Shi<yang.shi@windriver.com>
---
  arch/mips/lib/delay.c |    6 +++++-
  1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/arch/mips/lib/delay.c b/arch/mips/lib/delay.c
index 6b3b1de..dc38064 100644
--- a/arch/mips/lib/delay.c
+++ b/arch/mips/lib/delay.c
@@ -41,7 +41,11 @@ EXPORT_SYMBOL(__delay);

  void __udelay(unsigned long us)
  {
-       unsigned int lpj = current_cpu_data.udelay_val;
+       unsigned int lpj;
+
+       preempt_disable();
+       lpj = current_cpu_data.udelay_val;
+       preempt_enable();

        __delay((us * 0x000010c7ull * HZ * lpj)>>  32);
  }
This doesn't seem like the best approach.

Perhaps we should either use raw_current_cpu_data and no preempt_disable(), or if we are concerned about migrating to a CPU with a different lpj value, move the preempt_enable after the call to __delay().

Thanks David.

Yes, actually I also has this concern, so this patch is just a rough fix. And I tried raw_current_cpu_data as well, but I'm not sure if it's safe or not. Another proposal is to change cpu_data and current_cpu_data to per CPU variables, of course this is a big change.

Regards,
Yang

David Daney






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