> 2023年5月3日 01:28，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Tue, May 02, 2023 at 11:51:33PM +0800, Alan Huang wrote:
>>
>>
>>> 2023年5月2日 03:02，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>
>>> On Mon, May 01, 2023 at 07:09:49PM +0800, Alan Huang wrote:
>>>>
>>>>> 2023年5月1日 下午7:05，Alan Huang <mmpgouride@xxxxxxxxx> 写道：
>>>>>
>>>>> Hi,
>>>>>
>>>>> I’m reading section 9.5.4.9, in the second to the last paragraph, there is:
>>>>>
>>>>> Before the first horizontal line, all readers see the leftmost myconfig structure,
>>>>> and after the second horizontal line, all readers will see the rightmost structure.
>>>>> Between the two lines, that is, during the grace period, different readers might see different objects
>>>>>
>>>>> I’m a little confused, in Figure 9.29, the xchg is before the first horizontal line, in my understanding, if the
>>>>> reader’s reading is before the xchg, it will read the leftmost myconfg structure.
>>>>
>>>> And if reader’s reading is after the xchg, it will see the rightmost structure, and because xchg is before the first horizontal line,
>>>> Readers before the first horizontal line may also see the rightmost structure.
>>>>
>>>>>
>>>>> So, are there some conflicts between the paragraph and the figure?
>>>>>
>>>>> Please correct me if I’m wrong.
>>>
>>> Consider the point in time just before the left-hand reader's
>>> rcu_read_unlock() and just after the right-hand reader's rcu_read_lock().
>>> These two readers are both running at that time, and one of those readers
>>> sees the old state and the other sees the new state.
>>
>> The sentence that may lead to conflict is this one from the original text:
>>
>> Before the first horizontal line, all readers see the leftmost myconfig structure.
>>
>> There may be a situation where:
>>
>> mcp = xchg(&curconfig, mcp);
>> rcu_read_lock();
>> mcp = …
>> *cur_a = mcp->a;
>> -------------------------------------------------first horizontal line-------------------------------------------------
>>
>> So, the reader will see the rightmost myconfig structure although it is before the first horizontal line.
>
> Ah, I understand now, good catch and thank you! I updated that paragraph
> to the following, with your Reported-by:
>
> The two horizontal grey dashed lines represent the period of time
> during which different readers get different results, however,
> each reader will see one and only one of the two objects. All
> readers that end before the first horizontal line will see the
How about this:
mcp = xchg(&curconfig, mcp);
rcu_read_lock();
mcp = …
*cur_a = mcp->a;
rcu_read_unlock();
-------------------------------------------------first horizontal line-------------------------------------------------
The reader above end before the first horizontal line, but still get the rightmost myconfig structure.
The point is that there is a time period between xchg and the first horizontal line during which a reader
will see the rightmost myconfig structure.
One possible solution is to move xchg behind the first horizontal line, assuming that I understand correctly.
> leftmost myconfig structure, and all readers that start after the
> second horizontal line will see the rightmost structure. Between
> the two lines, that is, during the grace period, different readers
> might see different objects, but as long as each reader loads the
> curconfig pointer only once, each reader will see a consistent
> view of its myconfig structure.
>
> Does that help?
>
>> BTW, the original text says that black dotted arrows pointing from an older event to a newer event,
>> but there exists two black dotted arrows in the wrong direction:
>>
>> 1.The arrow pointing from synchronize_rcu() to left-hand rcu_read_lock()
>> 2.The arrow pointing right-hand rcu_read_lock() to synchronize_rcu()
>>
>> The directions of the two arrows above may need to be adjusted. I tried to modify this, but after a few hours, I still can’t figure out the svg...
>
> The trick is to use an SVG editor. For better or for worse, I use
> inkscape. But easy for me to change, so I did, again, with your
> Reported-by.
>
> The trick is to flip each arrow horizontally and then vertically, which
> reverses the direction without having to test one's manual dexterity,
> which I lack in abundance. ;-)
Thanks for the tricks, will try to use it!
Thanks,
Alan
>
> Thanx, Paul
>
>> Thanks,
>> Alan
>>
>>>
>>> Does that help, or am I missing the point of your question?
>>>
>>> Thanx, Paul
