synchronized (this[.classinfo]) in druntime and phobos

Regan Heath regan at
Tue May 29 05:07:29 PDT 2012

On Tue, 29 May 2012 01:08:59 +0100, Jonathan M Davis <jmdavisProg at>  

> On Tuesday, May 29, 2012 01:54:59 Alex Rønne Petersen wrote:
>> On 29-05-2012 01:41, Jonathan M Davis wrote:
>> > On Tuesday, May 29, 2012 01:35:23 Alex Rønne Petersen wrote:
>> >> I don't think arguing about them makes sense at this point. Way too  
>> much
>> >> code would break if we changed the semantics. I'd consider it a  
>> mistake
>> >> and a lesson learned, rather.
>> >>
>> >> But I take it you agree that synchronized (this) and similar
>> >> "uncontrollable" synchronization blocks should be avoided?
>> >
>> > I'm not an expert on threading stuf, but it would be my opinion that  
>> if
>> > you're not intending to protect the entire class with locks that it  
>> makes
>> > no sense to lock on the class itself. You're locking for something
>> > specific, in which case, your sychronized block should be locking on
>> > something else specific to what you're trying to protect. Certainly,
>> > that's how I'd approach it with mutexes. You don't have a mutex for an
>> > entire class unless it's actually used for all of the class'  
>> functions.
>> > Rather, you use mutexes specific to what you're trying to protect.
>> >
>> > - Jonathan M Davis
>> Right, but even if you really *are* protecting the entire class, you can
>> still create mysterious deadlocks if users of your code lock on your
>> class. So I'm arguing that no matter the use case, never lock on a
>> 'this' reference exposed outside of some API layer.
> Well, it seems pretty abysmal to me to be locking on something that you  
> don't control. Making a mutex that your class used public would just be  
> stupid.

Interestingly this is what C#s Array type does with SyncRoot  

> With synchronized classes/functions, you're basically creating and using  
> an
> implicit mutex for the whole class, which would then be the same as if  
> you locked it at the beginning of every member function call and  
> unlocked it at its end, which doesn't expose the mutex at all. So, I  
> don't really see te
> problem there would have to study the matter more to see what the issue  
> there is.

According to the docs here:

A synchronized function locks "this" and as "this" is exposed publicly...   
In the following code the "lock" statement and "synchronized void bar"  
lock the same mutex.

class Foo {
   synchronized void bar() { ...statements... }

void main()
   Foo foo = new Foo();

> But locking on another class rather than something specifically intended  
> as a mutex does seem to me like it's asking for trouble.

Yep.  It commonly arises where you have a class/object which is either not  
synchronized itself (because it might be used in single threaded  
situations and you want performance) like a collection class for example,  
or is synchronized but you need to lock a sequence of member function  
calls i.e. a lookup and insert on the collection.  What happens then, is  
people just call lock(<object>) and end up locking the same mutex as the  
class does internally.  What happens next to cause a deadlock is that  
inside that lock they call one or more functions or methods which  
internally call methods on another synchronized object.  This results in a  
locking pattern of object1, object2.  In another piece of code, running in  
another thread, they do something similar which results in a locking  
pattern of object2, object1 and eventually these threads will deadlock  
each other.


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