Re: [PATCH] rcuupdate: Do a single rhp->func read in rcu_head_after_call_rcu

[Neeraj Upadhyay <neeraju@codeaurora.org>]

On 3/11/19 9:18 PM, Paul E. McKenney wrote:
> On Mon, Mar 11, 2019 at 05:28:03PM +0530, Neeraj Upadhyay wrote:
>> Read rhp->func pointer in rcu_head_after_call_rcu() only once,
>> to avoid warning in the case, where call_rcu() happens between
>> two reads of rhp->func.
>>
>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> 
> This would more gracefully handle racing rcu_head_after_call_rcu()
> with call_rcu().
> 
> But this thing is not yet used, so let's see what Neil Brown says.
> If he isn't going to use it, my thought is to instead just remove
> this.
> 
> 							Thanx, Paul
> 

Agree, makes sense.

Thanks
Neeraj

>> ---
>>   include/linux/rcupdate.h | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> index 6cdb1db..174768e 100644
>> --- a/include/linux/rcupdate.h
>> +++ b/include/linux/rcupdate.h
>> @@ -878,9 +878,10 @@ static inline void rcu_head_init(struct rcu_head *rhp)
>>   static inline bool
>>   rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
>>   {
>> -	if (READ_ONCE(rhp->func) == f)
>> +	rcu_callback_t func = READ_ONCE(rhp->func);
>> +	if (func == f)
>>   		return true;
>> -	WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
>> +	WARN_ON_ONCE(func != (rcu_callback_t)~0L);
>>   	return false;
>>   }
>>
>> -- 
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
>> member of the Code Aurora Forum, hosted by The Linux Foundation
>>
> 

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation