Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

[Joel Fernandes]

> On Dec 22, 2022, at 1:53 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> On Thu, Dec 22, 2022 at 01:19:06PM -0500, Joel Fernandes wrote:
>> 
>> 
>>>> On Dec 22, 2022, at 11:43 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>>> 
>>> On Thu, Dec 22, 2022 at 01:40:10PM +0100, Frederic Weisbecker wrote:
>>>>> On Wed, Dec 21, 2022 at 12:11:42PM -0500, Mathieu Desnoyers wrote:
>>>>> On 2022-12-21 06:59, Frederic Weisbecker wrote:
>>>>>>> On Tue, Dec 20, 2022 at 10:34:19PM -0500, Mathieu Desnoyers wrote:
>>>>> [...]
>>>>>>> 
>>>>>>> The memory ordering constraint I am concerned about here is:
>>>>>>> 
>>>>>>> * [...] In addition,
>>>>>>> * each CPU having an SRCU read-side critical section that extends beyond
>>>>>>> * the return from synchronize_srcu() is guaranteed to have executed a
>>>>>>> * full memory barrier after the beginning of synchronize_srcu() and before
>>>>>>> * the beginning of that SRCU read-side critical section. [...]
>>>>>>> 
>>>>>>> So if we have a SRCU read-side critical section that begins after the beginning
>>>>>>> of synchronize_srcu, but before its first memory barrier, it would miss the
>>>>>>> guarantee that the full memory barrier is issued before the beginning of that
>>>>>>> SRCU read-side critical section. IOW, that memory barrier needs to be at the
>>>>>>> very beginning of the grace period.
>>>>>> 
>>>>>> I'm confused, what's wrong with this ?
>>>>>> 
>>>>>> UPDATER                  READER
>>>>>> -------                  ------
>>>>>> STORE X = 1              STORE srcu_read_lock++
>>>>>> // rcu_seq_snap()        smp_mb()
>>>>>> smp_mb()                 READ X
>>>>>> // scans
>>>>>> READ srcu_read_lock
>>>>> 
>>>>> What you refer to here is only memory ordering of the store to X and load
>>>>> from X wrt loading/increment of srcu_read_lock, which is internal to the
>>>>> srcu implementation. If we really want to model the provided high-level
>>>>> memory ordering guarantees, we should consider a scenario where SRCU is used
>>>>> for its memory ordering properties to synchronize other variables.
>>>>> 
>>>>> I'm concerned about the following Dekker scenario, where synchronize_srcu()
>>>>> and srcu_read_lock/unlock would be used instead of memory barriers:
>>>>> 
>>>>> Initial state: X = 0, Y = 0
>>>>> 
>>>>> Thread A                   Thread B
>>>>> ---------------------------------------------
>>>>> STORE X = 1                STORE Y = 1
>>>>> synchronize_srcu()
>>>>>                          srcu_read_lock()
>>>>>                          r1 = LOAD X
>>>>>                          srcu_read_unlock()
>>>>> r0 = LOAD Y
>>>>> 
>>>>> BUG_ON(!r0 && !r1)
>>>>> 
>>>>> So in the synchronize_srcu implementation, there appears to be two
>>>>> major scenarios: either srcu_gp_start_if_needed starts a gp or expedited gp,
>>>>> or it uses an already started gp/expedited gp. When snapshotting with
>>>>> rcu_seq_snap, the fact that the memory barrier is after the ssp->srcu_gp_seq
>>>>> load means that it does not order prior memory accesses before that load.
>>>>> This sequence value is then used to identify which gp_seq to wait for when
>>>>> piggy-backing on another already-started gp. I worry about reordering
>>>>> between STORE X = 1 and load of ssp->srcu_gp_seq, which is then used to
>>>>> piggy-back on an already-started gp.
>>>>> 
>>>>> I suspect that the implicit barrier in srcu_read_lock() invoked at the
>>>>> beginning of srcu_gp_start_if_needed() is really the barrier that makes
>>>>> all this behave as expected. But without documentation it's rather hard to
>>>>> follow.
>>>> 
>>>> Oh ok I see now. It might be working that way by accident or on forgotten
>>>> purpose. In any case, we really want to add a comment above that
>>>> __srcu_read_lock_nmisafe() call.
>>> 
>>> Another test for the safety (or not) of removing either D or E is
>>> to move that WRITE_ONCE() to follow (or, respectively, precede) the
>>> adjacent scans.
>> 
>> Good idea, though I believe the MBs that the above talk about are not the flip ones. They are the ones in synchronize_srcu() beginning and end, that order with respect to grace period start and end.
>> 
>> So that (flipping MBs) is unrelated, or did I miss something?
> 
> The thought is to manually similate in the source code the maximum
> memory-reference reordering that a maximally hostile compiler and CPU
> would be permitted to carry out.  So yes, given that there are other
> memory barriers before and after, these other memory barriers limit how
> far the flip may be moved in the source code.
> 
> Here I am talking about the memory barriers associated with the flip,
> but the same trick can of course be applied to other memory barriers.
> In general, remove a given memory barrier and (in the source code)
> maximally rearrange the memory references that were previously ordered
> by the memory barrier in question.
> 
> Again, the presence of other memory barriers will limit the permitted
> maximal source-code rearrangement.


Makes sense if the memory barrier is explicit. In this case, the memory barriers are implicit apparently, with a srcu_read_lock() in the beginning of synchronize_rcu() having the implicit / indirect memory barrier. So I am not sure if that can be implemented without breaking SRCU readers.

Thanks.




> 
>                            Thanx, Paul