On Saturday, October 28, 2017 at 9:24:56 AM UTC-4, T L wrote: > > > > On Saturday, October 28, 2017 at 8:45:00 AM UTC-4, Axel Wagner wrote: >> >> No. The channel operations do not act like a fence, but like a… channel, >> through which happens-before relationships can be transported. The crucial >> point in the example you quoted is, that the channel-communication creates >> a happens-before relationship between the two goroutines, which then >> connects the happens-before relationships between channel-send/write and >> channel-read/print. >> >> Let's say, hypothetically, the happens-before relationship for >> channel-operations was turned around: The read happens before the send. >> Conceptually, they happen simultaneously and just from a local perspective, >> both operations would still look just as much as a "fence". But suddenly, >> there wouldn't be a happens-before relationship between the write and the >> print: >> write ≤ channel-send ≥ channel-read ≤ print >> > > There is not a constant order between the send and the receive. > For an unbufferred channel, the guaranteed orders include: > 1. the send must happen before the completion of the receive, > 2. the receive must happen before the completion of the send. > In other words, the completion of the receive and the the completion of > the send happen at the same time, > they should be viewed as the same event. Both of the send and receive > happen before the event. > So, at least for the two involved goroutines, the completion event must > act as a fence to guarantee the memory model. > > In theory, the completion event is able to NOT act as a fence. > But I just doubt if it is possible from the implementation view. >
Here, I mean "In theory, the completion event is able to NOT act as a fence *for other goroutines*. But I just doubt if it is possible from the implementation view." > > >> >> You *have* to look at relationships between goroutines. And you need (or >> rather it's incredibly sensible) to take the causality-lens when >> understanding the memory model; it was chosen for a reason. The memory >> model is about establishing causal relationships between events in >> different goroutines and about the direction of that causality. It only >> makes promises about what GR A observes about GR B *if* there is an edge >> between them and only *if* that edge is correctly oriented. >> >> On Sat, Oct 28, 2017 at 1:45 PM, T L <tapi...@gmail.com> wrote: >> >>> The first example in the "Channel communication" section, it says >>> >>> The write to a happens before the send on c, which happens before the >>> corresponding receive on c completes, which happens before the print. >>> >>> From the description, I feel that "the write to a happens before the >>> send on c" is not caused by >>> "the send on c happens before the corresponding receive on c completes". >>> And "the corresponding receive on c completes happens before the print" >>> is also independent to >>> "the send on c happens before the corresponding receive on c completes". >>> >>> So it looks channel read and write operations really act as a fance to >>> avoid exchanging relative orders of statements before and after the fence. >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "golang-nuts" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to golang-nuts...@googlegroups.com. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
