There has no Mutex.Trylock() implemention or similar method ?

[Michael Liu]

I've a race scenario used with Mutex.Lock with Lock() and Unlock(). now multi-routines try to lock the resource and start a few logical code if the Lock.Lock() succesfully. other routines don't need to block util Lock.Unlock() that they can do the above logicals with next time or in future(logical may changes some variables and those variables' change could be see with latency). That looks like a Trylock() implemetion.

I browsed the discussion https://groups.google.com/forum/#!searchin/golang-nuts/trylock%7Csort:relevance/golang-nuts/MTaJNZ49u60/ycc7UHgjLwgJ and guess Trylock() may the best way ...

var lock sync.Mutex

func change(vvv int) bool {

     if lock.Trylock() {

          // execute codes if we trylock successfully

          bla bla.......

          usage := vvv .......

          bla bla.......

          return true

     } else { 

          // someone is locking the resource. we skip the 

          // change(vvvv) call. and take a chance in next

          // time. the context of this method we can drop

          // directly

        

          return false

     }

}

routines := 16

for ;routines != 0; routines-- {

    go func() {

        // infinite loop 

        var i = 0

        for {

            // discard the return value of change()

            change(i)

            i++

        }

    }()

}

[Jan Mercl]

On Fri, Oct 21, 2016 at 3:05 PM Michael Liu <gugem...@gmail.com> wrote:

It's possible to model a lock using a semafor. Semafor can be implemented by a channel. The select statement can be then used for the "tryLock" operation.

--

-j

[Roberto Zanotto]

Maybe you can solve this with atomics.
You keep an int32 that acts as mutex (0 is unocked, 1 is locked),

the Trylock is done with atomic.CompareAndSwapInt32(mu, 0, 1) and the Unlock with atomic.StoreInt32(mu, 0).

This gives you a Trylock, but you lose the standard (blocking) Lock.

You may have to put your "usage" variable inside an atomic.Value,

if some goroutines want to read it while another is modifying it.

Be sure to test your code with -race.

[Henrik Johansson]

There was a long thread just recently about SpinLocks and the memory model.

There are several implementations on github. I cannot vouch for the quality of any of them.

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[adon...@google.com]

On Friday, 21 October 2016 09:05:10 UTC-4, Michael Liu wrote:

I've a race scenario used with Mutex.Lock with Lock() and Unlock(). now multi-routines try to lock the resource and start a few logical code if the Lock.Lock() succesfully. other routines don't need to block util Lock.Unlock() that they can do the above logicals with next time or in future(logical may changes some variables and those variables' change could be see with latency). That looks like a Trylock() implemetion.

One reason the TryLock method does not exist is that its behavior cannot be expressed without reference to some notion of goroutine identity.  That is, its doc comment would read "succeeds immediately if the current goroutine already holds the lock".  The designers of the language have strived to avoid making goroutine state relevant to the behavior of any function since it makes programs had to reason about and prevents programmers from freely moving work to a different goroutine.

Another reason is described in Chapter 9 of our book (gopl.io): "There is a good reason Go’s mutexes are not re-entrant. The purpose of a mutex is to ensure that certain invariants of the

shared variables are maintained at critical points during program execution. One of the invariants is “no goroutine is accessing the shared variables,” but there may be additional invariants specific to the data structures that the mutex guards. When a goroutine acquires a mutex lock, it may assume that the invariants hold. While it holds the lock, it may update the shared variables so that the invariants are temporarily violated. However, when it releases the lock, it must guarantee that order has been restored and the invariants hold once again. Although a re-entrant mutex would ensure that no other goroutines are accessing the shared variables, it cannot protect the additional invariants of those variables."

[John Souvestre]

I understand what you are saying with regard to a reentrant type of mutex.  But how about a non-reentrant mutex, as Go currently has?  Borrowing from an old message by Luke Scott:

 

> I was looking at the sync package's Lock function, and it does

 > almost the same thing.

> It looks like a TryLock function would look like this:

>

 > func (m *Mutex) TryLock() bool {

>     return atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked)

> }

>

 > Is this correct? If so, what's the problem?

 

I don’t recall seeing an answer to his question, however.

 

John

    John Souvestre - New Orleans LA

--

[dja...@gmail.com]

Hi,

Trylock is simple as: https://play.golang.org/p/rpy3Kg1KWW

Regards,

Djadala

[John Souvestre]

Take a look at https://github.com/LK4D4/trylock/blob/master/trylock.go .

 

I believe that it is easier and performs better.

 

John

    John Souvestre - New Orleans LA

 

From: golan...@googlegroups.com [mailto:golan...@googlegroups.com] On Behalf Of dja...@gmail.com
Sent: 2016 October 23, Sun 10:30
To: golang-nuts
Subject: [go-nuts] Re: There has no Mutex.Trylock() implemention or similar method ?

 

Hi,

--

[adon...@google.com]

On Sunday, 23 October 2016 16:17:29 UTC-4, John Souvestre wrote:

Take a look at https://github.com/LK4D4/trylock/blob/master/trylock.go .

 

I believe that it is easier and performs better.

Yes, this looks like a sound solution if you don't need the re-entrant behavior.