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

[Yang Shi <yang.shi@windriver.com>]

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
>
>