Re: [RFC] tools/memory-model: Rule out OOTA

Am 1/16/2025 um 7:40 PM schrieb Paul E. McKenney:
> On Mon, Jan 13, 2025 at 02:04:58PM -0800, Paul E. McKenney wrote:
>> On Fri, Jan 10, 2025 at 05:21:59PM +0100, Jonas Oberhauser wrote:
>>> Am 1/10/2025 um 3:54 PM schrieb Paul E. McKenney:
>>>> On Thu, Jan 09, 2025 at 07:35:19PM +0100, Jonas Oberhauser wrote:
>>>>> Am 1/9/2025 um 6:54 PM schrieb Paul E. McKenney:
>>>>>> On Wed, Jan 08, 2025 at 08:17:51PM +0100, Jonas Oberhauser wrote:
> 
> [ . . . ]
> 
>>>>> I currently can not come up with an example where there would be a
>>>>> (semantic) control dependency from a load to a store that is not in the arm
>>>>> of an if statement (or a loop / switch of some form with the branch
>>>>> depending on the load).
>>>>>
>>>>> I think the control dependency is just a red herring. It is only there to
>>>>> avoid the data race.
>>>>
>>>> Well, that red herring needs to have a companion fish to swim with in
>>>> order to enforce ordering, and I am not seeing that companion.
>>>>
>>>> Or am I (yet again!) missing something subtle here?
>>>
>>> It makes more sense to think about how people do message passing (or
>>> seqlock), which might look something like this:
>>>
>>>    [READ_ONCE]
>>>    rmb
>>>    [plain read]
>>>
>>> and
>>>
>>>    [plain write]
>>>    wmb
>>>    [WRITE_ONCE]
>>>
>>>
>>> Clearly LKMM says that there is some sort of order (not quite happens-before
>>> order though) between the READ_ONCE and the plain read, and between the
>>> plain write and the WRITE_ONCE. This order is clearly defined in the data
>>> race definition, in r-pre-bounded and w-post-bounded.
>>>
>>> Now consider
>>>
>>>    [READ_ONCE]
>>>    rmb
>>>    [plain read]
>>>    // some code that creates order between the plain accesses
>>>    [plain write]
>>>    wmb
>>>    [WRITE_ONCE]
>>>
>>> where for some specific reason we can discern that the compiler can not
>>> fully eliminate/move across the barrier either this specific plain read, nor
>>> the plain write, nor the ordering between the two.
>>>
>>> In this case, is there order between the READ_ONCE and the WRITE_ONCE, or
>>> not? Of course, we know current LKMM says no. I would say that in those very
>>> specific cases, we do have ordering.
>>
>> Agreed, for LKMM to deal with seqlock, the read-side critical section
>> would need to use READ_ONCE(), which is a bit unnatural.  The C++
>> standards committee has been discussing this for some time, as that
>> memory model also gives data race in that case.
>>
>> But it might be better to directly model seqlock than to try to make
>> LKMM deal with the underlying atomic operations.
> 
> Maybe I should give an example approach, perhaps inspiring a better
> approach.
> 
> o	Model reader-writer locks in LKMM, including a relaxed
> 	write-lock-held primitive.
> 
> o	Model sequence locks in terms of reader-writer locks:
> 
> 	o	The seqlock writer maps to write lock.
> 
> 	o	The seqlock reader maps to read lock, but with a
> 		write-lock-held check.	If the write lock is held at
> 		that point, the seqlock tells the caller to retry.
> 
> 		Please note that the point is simply to exercise
> 		the failure path.
> 
> 	o	If a value is read in the seqlock reader and used
> 		across a "you need to retry" indication, that
> 		flags a seqlock data race.
> 
> But is there a better way?
> 

You need to be careful with those hb edges. The reader critical section 
does not have to happen-before the writer critical section, as would 
with an actual read-write lock.

I think the solution would have to be along changing the definition of 
r-post-bounded.
The read_enter() function reads from a write_exit() and establishes hb.
The read_exit() function also reads from a write_exit(), if the same as 
the matching read_enter(), then it returns success, otherwise failure.

Reads po-before a successful read_exit() are bounded with regards to 
subsequent write_enter() on the same lock.

Best wishes,
   jonas