Re: [PATCH 3/3] smp/ipi:Remove check around csd lock in handler for smp_call_function variants

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 07/06/2013 11:15 AM, Wang YanQing wrote:
> On Fri, Jul 05, 2013 at 09:57:21PM +0530, Preeti U Murthy wrote:
>> call_single_data is always locked by all callers of
>> arch_send_call_function_single_ipi() or
>> arch_send_call_function_ipi_mask() which results in execution of
>> generic_call_function_interrupt() handler.
>>
>> Hence remove the check for lock on csd in generic_call_function_interrupt()
>> handler, before unlocking it.
> 
> I can't find where is the generic_call_function_interrupt :)

Sorry about this error :)
> 
>> Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
>> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
>> Cc: Ingo Molnar <mingo@elte.hu>
>> Cc: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
>> Cc: srivatsa.bhat@linux.vnet.ibm.com
>> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Rusty Russell <rusty@rustcorp.com.au
>> ---
>>
>>  kernel/smp.c |   14 +-------------
>>  1 file changed, 1 insertion(+), 13 deletions(-)
>>
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index b6981ae..d37581a 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -181,25 +181,13 @@ void generic_smp_call_function_single_interrupt(void)
>>  
>>  	while (!list_empty(&list)) {
>>  		struct call_single_data *csd;
>> -		unsigned int csd_flags;
>>  
>>  		csd = list_entry(list.next, struct call_single_data, list);
>>  		list_del(&csd->list);
>>  
>> -		/*
>> -		 * 'csd' can be invalid after this call if flags == 0
>> -		 * (when called through generic_exec_single()),
>> -		 * so save them away before making the call:
>> -		 */
>> -		csd_flags = csd->flags;
>> -
> 
> You haven't mention this change in the ChangeLog, don't do it.

Right, I will include it in the changelog.

> I can't see any harm to remove csd_flags, but I hope others
> check it again.
> 
>>  		csd->func(csd->info);
>>  
>> -		/*
>> -		 * Unlocked CSDs are valid through generic_exec_single():
>> -		 */
>> -		if (csd_flags & CSD_FLAG_LOCK)
>> -			csd_unlock(csd);
>> +		csd_unlock(csd);
> 
> I don't like this change, I think check CSD_FLAG_LOCK 
> to make sure we really need csd_unlock is good.

Ideally it should be under a WARN_ON(). csd_unlock() has that WARN_ON().
Unlocking a parameter which is not locked should be seen as a bug, which
the above code is not doing. In fact it avoids it being reported as a bug.

> 
> Just like you can't know who and how people will use the
> API, so some robust check code is good.
> 

Regards
Preeti U Murthy