On Thu, Nov 3, 2016 at 11:30 PM, 刘桂祥 <liuguixiang...@gmail.com> wrote: > > very thanks , I am unfamiliar with the cpu out-of-order execution , but > I doubt if my three line code is related ,it will reorder it ??
I want to be clear that the real problem is not out-of-order execution, it is memory ordering. On modern multi-core processors there is no guarantee that one core will see memory writes in the order they are done by a different core. Making that guarantee requires an explicit memory barrier instruction, and there is no such instruction in your example program. Ian > 在 2016年11月4日星期五 UTC+8下午1:38:12，Ian Lance Taylor写道： >> >> On Thu, Nov 3, 2016 at 10:28 PM, 刘桂祥 <liuguix...@gmail.com> wrote: >> > >> > In my write goroutine I don't modify the map key (data structure) but >> > set >> > the variable point to another map address ,this is a pointer assignment >> > and >> > is atomic >> >> You're right, my previous reply was incorrect. My apologies. The >> real reason your code is incorrect is more subtle. >> >> A map is a pointer to a complex data structure. Setting values in a >> map is a series of memory writes. From the point of view of the >> processor doing the writes, they all appear in order. From the >> pointer of view of a different processor, they need not. It is >> possible the the other processor to see the final write, to gMap, >> before it sees the other writes, setting the values in the map. The >> read from gMap will then be trying to access an incomplete data >> structure. >> >> In order to force all the writes to be visible to the reading >> processor, you need to use a sync.Mutex, a channel operation, or a >> sync/atomic.Store function. >> >> There is a simple rule to follow when it comes to communicating >> between processes: always use channels or locks. Don't try to be >> clever. Please read >> >> https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong. >> >> Ian >> >> > 在 2016年11月4日星期五 UTC+8下午1:22:20，Ian Lance Taylor写道： >> >> >> >> On Thu, Nov 3, 2016 at 10:19 PM, 刘桂祥 <liuguix...@gmail.com> wrote: >> >> > can you explain why whis ? >> >> >> >> A map is basically a pointer to a complex data structure. Setting a >> >> value in a map changes that data structure. If one goroutine is >> >> reading from the data structure while a different goroutine is writing >> >> to the data structure, the results are completely unpredictable. In >> >> the worst case they could even cause the program to crash. >> >> >> >> Ian >> >> >> >> >> >> > 在 2016年11月4日星期五 UTC+8下午1:16:39，Ian Lance Taylor写道： >> >> >> >> >> >> On Thu, Nov 3, 2016 at 8:37 PM, 刘桂祥 <liuguix...@gmail.com> wrote: >> >> >> > // example.go >> >> >> > >> >> >> > package main >> >> >> > >> >> >> > var gMap = make(map[int]int) >> >> >> > >> >> >> > func w() { >> >> >> > temp := make(map[int]int) >> >> >> > temp[1] = 100 >> >> >> > temp[2] = 200 >> >> >> > gMap = temp // Does the compiler or cpu will reorder >> >> >> > temp[1]=100, >> >> >> > temp[2]=200, gMap=temp ?? >> >> >> > } >> >> >> > >> >> >> > func r() { >> >> >> > local := gMap >> >> >> > println(local[1], local[2]) >> >> >> > } >> >> >> > >> >> >> > func main() { >> >> >> > >> >> >> > go w() >> >> >> > go r() >> >> >> > >> >> >> > // ... >> >> >> > } >> >> >> > >> >> >> > I have one goroutine to read the map and one goroutine to rewrite >> >> >> > the >> >> >> > global >> >> >> > map variable does this safe ?? >> >> >> >> >> >> No. Use a lock. >> >> >> >> >> >> Ian >> >> > >> >> > -- >> >> > 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...@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. -- 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.
