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

[Joel Fernandes]

> On Dec 22, 2022, at 7:40 AM, Frederic Weisbecker <frederic@xxxxxxxxxx> 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.

Agreed on the analysis and the need for comments, thanks!

I think we also ought to document some more cases like, how the memory barriers here relate to the other memory barrier A inside the scan loop. But I guess for me I need to understand the GP guarantee ordering first before attempting documenting that aspect. Oh well, thank God for holidays. ;-)

Thanks,

 - Joel



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