On Wed, May 03, 2023 at 12:31:05PM +0800, Alan Huang wrote: > > > > 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. Yes, it would in some sense be more accurate to run that horizontal line right through the "mcp = xchg(&curconfig, mcp);". But can you do that so that the diagram looks OK and is legible? Thanx, Paul > > 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 > >
