Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer() data race and concurrency bug

[Joel Fernandes]

On 3/4/2024 2:44 PM, Paul E. McKenney wrote:
> On Mon, Mar 04, 2024 at 02:10:09PM -0500, Joel Fernandes wrote:
>>
>>
>> On 3/4/2024 12:14 PM, Paul E. McKenney wrote:
>>> On Mon, Mar 04, 2024 at 11:19:21AM -0500, Joel Fernandes wrote:
>>>>
>>>>
>>>> On 3/4/2024 5:54 AM, linke li wrote:
>>>>> Some changes are done to fix a data race in commit 202489101f2e ("rcutorture: Fix rcu_torture_one_read()/rcu_torture_writer() data race")
>>>>>
>>>>>  {
>>>>>  	int i;
>>>>>
>>>>> -	i = rp->rtort_pipe_count;
>>>>> +	i = READ_ONCE(rp->rtort_pipe_count);
>>>>>  	if (i > RCU_TORTURE_PIPE_LEN)
>>>>>  		i = RCU_TORTURE_PIPE_LEN;
>>>>>  	atomic_inc(&rcu_torture_wcount[i]);
>>>>> -	if (++rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
>>>>> +	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
>>>>> +	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
>>>>>  		rp->rtort_mbtest = 0;
>>>>>  		return true;
>>>>>  	}
>>>>>
>>>>> But ++rp->rtort_pipe_count is meant to add itself by 1, not give i+1 to
>>>>> rp->rtort_pipe_count, because rp->rtort_pipe_count may write by
>>>>> rcu_torture_writer() concurrently.
>>>>>
>>>>> Also, rp->rtort_pipe_count in the next line should be read using
>>>>> READ_ONCE() because of data race.
>>>>>
>>>>> Signed-off-by: linke li <lilinke99@xxxxxx>
>>>>> ---
>>>>>  kernel/rcu/rcutorture.c | 4 ++--
>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>>>>> index 7567ca8e743c..00059ace4fd5 100644
>>>>> --- a/kernel/rcu/rcutorture.c
>>>>> +++ b/kernel/rcu/rcutorture.c
>>>>> @@ -465,8 +465,8 @@ rcu_torture_pipe_update_one(struct rcu_torture *rp)
>>>>>  	if (i > RCU_TORTURE_PIPE_LEN)
>>>>>  		i = RCU_TORTURE_PIPE_LEN;
>>>>>  	atomic_inc(&rcu_torture_wcount[i]);
>>>>> -	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
>>>>> -	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
>>>>> +	WRITE_ONCE(rp->rtort_pipe_count, rp->rtort_pipe_count + 1);
>>>>> +	if (READ_ONCE(rp->rtort_pipe_count) >= RCU_TORTURE_PIPE_LEN) {
>>>>
>>>> I want to say, I am not convinced with the patch because what's wrong with
>>>> writing to an old index?
>>>>
>>>> You win/lose the race anyway, say the CPU executed the WRITE_ONCE() a bit too
>>>> early/late and another WRITE_ONCE() lost/won, regardless of whether you wrote
>>>> the "incremented i" or "the increment from the latest value of pipe_count".
>>>>
>>>> Anyway, a slightly related/different question:
>>>>
>>>> Should that:
>>>> WRITE_ONCE(rp->rtort_pipe_count, rp->rtort_pipe_count + 1);
>>>>
>>>> Be:
>>>> WRITE_ONCE(rp->rtort_pipe_count, READ_ONCE(rp->rtort_pipe_count) + 1);
>>>>
>>>> ?
>>>
>>> Thank you both!
>>>
>>> At first glance, I would argue for something like this:
>>>
>>> ------------------------------------------------------------------------
>>>
>>> static bool
>>> rcu_torture_pipe_update_one(struct rcu_torture *rp)
>>> {
>>> 	int i;
>>> 	struct rcu_torture_reader_check *rtrcp = READ_ONCE(rp->rtort_chkp);
>>>
>>> 	if (rtrcp) {
>>> 		WRITE_ONCE(rp->rtort_chkp, NULL);
>>> 		smp_store_release(&rtrcp->rtc_ready, 1); // Pair with smp_load_acquire().
>>> 	}
>>> 	i = READ_ONCE(rp->rtort_pipe_count) + 1;
>>> 	if (i > RCU_TORTURE_PIPE_LEN)
>>> 		i = RCU_TORTURE_PIPE_LEN;
>>> 	atomic_inc(&rcu_torture_wcount[i]);
>>> 	WRITE_ONCE(rp->rtort_pipe_count, i);
>>> 	if (i >= RCU_TORTURE_PIPE_LEN) {
>>> 		rp->rtort_mbtest = 0;
>>> 		return true;
>>> 	}
>>> 	return false;
>>> }
>>>
>>> ------------------------------------------------------------------------
>>>
>>> That is, move the increment to the read and replace the re-read with
>>> the value "i" that was just written.
>>
>> But that changes the original logic as well? It looks like with the above
>> change, you're now writing to rcu_torture_wcount[READ_ONCE(rp->rtort_pipe_count)
>> + 1] instead of rcu_torture_wcount[READ_ONCE(rp->rtort_pipe_count)].
>>
>> I think that might break rcutorture, because there is an increment outside of
>> the first 2 entries in rcu_torture_wcount but not sure (need to look more).
> 
> Good point on never incrementing the zeroth entry!  Clearly I should
> have waited before replying.
> 
> How about the following?
> 
> ------------------------------------------------------------------------
> 
> static bool
> rcu_torture_pipe_update_one(struct rcu_torture *rp)
> {
> 	int i;
> 	struct rcu_torture_reader_check *rtrcp = READ_ONCE(rp->rtort_chkp);
> 
> 	if (rtrcp) {
> 		WRITE_ONCE(rp->rtort_chkp, NULL);
> 		smp_store_release(&rtrcp->rtc_ready, 1); // Pair with smp_load_acquire().
> 	}
> 	i = READ_ONCE(rp->rtort_pipe_count);
> 	if (i > RCU_TORTURE_PIPE_LEN)
> 		i = RCU_TORTURE_PIPE_LEN;
> 	atomic_inc(&rcu_torture_wcount[i]);
> 	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
> 	if (i + 1 >= RCU_TORTURE_PIPE_LEN) {
> 		rp->rtort_mbtest = 0;
> 		return true;
> 	}
> 	return false;
> }

Yes, this looks good to me. Thanks,
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>