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

[linke]

Thank you both. This looks good to me. I will send a new patch.

> 2024年3月5日 04:47，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
> 
> On Mon, Mar 04, 2024 at 03:13:10PM -0500, Joel Fernandes wrote:
>> 
>> 
>> 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>
> 
> Again, thank you.
> 
> linke li, does this approach work for you?  If so, would you be willing to
> send a new patch along these lines?  If it does not work, what additional
> problems do you see?
> 
> 							Thanx, Paul