Issue with Mutual Execution of Concurrent Go Routines

Question

In my code there are three concurrent routines. I try to give a brief overview of my code,

Routine 1 {
do something

*Send int to Routine 2
Send int to Routine 3
Print Something
Print Something*

do something
}

Routine 2 {
do something

*Send int to Routine 1
Send int to Routine 3
Print Something
Print Something*

do something
}

Routine 3 {
do something

*Send int to Routine 1
Send int to Routine 2
Print Something
Print Something*

do something
}

main {
routine1
routine2
routine3
}

I want that, while codes between two do something (codes between two star marks) is executing, flow of control must not go to other go routines. For example, when routine1 is executing the events between two stars (sending and printing events), routine 2 and 3 must be blocked (means flow of execution does not pass to routine 2 or 3 from routine 1).After completing last print event, flow of execution may pass to routine 2 or 3.Can anybody help me by specifying, how can I achieve this ? Is it possible to implement above specification by WaitGroup ? Can anybody show me by giving a simple example how to implement above specified example by using WaitGroup. Thanks.

NB:May be this is repeat question of this. I tried by using that sync-lock mechanism, however, may be because I have a large code that's why I could not put lock-unlock properly, and it's creating deadlock situation (or may be my method is error producing). Can anybody help me by a simple procedure thus I can achieve this. I give a simple example of my code here where Here I want to put two prints and sending event inside mutex (for routine 1) thus routine 2 can't interrupt it. Can you help me how is it possible. One possible solution given, http://play.golang.org/p/-uoQSqBJKS which gives error.

Answer 1

Why do you want to do this?

The deadlock problem is, if you don't allow other goroutines to be scheduled, then your channel sends can't proceed, unless there's buffering. Go's channels have finite buffering, so you end up with a race condition on draining before they get sent on while full. You could introduce infinite buffering, or put each send in its own goroutine, but it again comes down to: why are you trying to do this; what are you trying to achieve?

Another thing: if you only want to ensure mutual exclusion of the three sets of code between *s, then yes, you can use mutexes. If you want to ensure that no code interrupts your block, regardless of where it was suspended, then you might need to use runtime.LockOSThread and runtime.UnlockOSThread. These are fairly low level and you need to know what you're doing, and they're rarely needed. Of you want there to be no other goroutines running, you'll have to have runtime.GOMAXPROCS(1), which is currently the default.

Answer 2

The problem in answering your question is that it seems no one understands what your problem really is. I see you're asking repeatedly about roughly the same, though no progress has been done. There's no offense in saying this. It's an attempt to help you by a suggestion to reformulate your problem in a way comprehensible to others. As a possible nice side effect, some problems do solve themselves while being explained to others in an understandable way. I've experienced that many times by myself.

Another hint could be in the suspicious mix of explicit syncing and channel communication. That doesn't mean the design is necessarily broken. It just doesn't happen in a typical/simple case. Once again, your problem might be atypical/non trivial.

Perhaps it's somehow possible to redesign your problem using only channels. Actually I believe that every problem involving explicit synchronization (in Go) could be coded while using only channels. That said, it is true some problems are written with explicit synchronization very easily. Also channel communication, as cheap as it is, is not as cheap as most synchronization primitives. But that could be looked after later, when the code works. If the "pattern" for some say sync.Mutex will visibly emerge in the code, it should be possible to switch to it and much more easy to do that when the code already works and hopefully has tests to watch your steps while making the adjustments.

Try to think about your goroutines like independently acting agents which:

• Exclusively own the data received from the channel. The language will not enforce this, you must deploy own's discipline.
• Don't anymore touch the data they've sent to a channel. It follows from first rule, but important enough to be explicit.
• Interact with other agents (goroutines) by data types, which encapsulate a whole unit of workflow/computation. This eliminates e.g. your earlier struggle with geting the right number of channel messages before the "unit" is complete.
• For every channel they use, it must be absolutely clear in before if the channel must be unbuffered, must be buffered for fixed number of items or if it may be unbound.
• Don't have to think (know) about what other agents are doing above getting a message from them if that is needed for the agent to do its own task - part of the bigger picture.

Using even such few rules of thumb should hopefully produce code which is more easy to reason about and which usually doesn't requires any other synchronization. (I'm intentionally ignoring performance issues of mission critical applications now.)