D const enables multi-reader synchronization

["Jérôme M. Berger"]

Robert Jacques wrote:
> On Mon, 14 Jun 2010 15:17:57 -0400, Tomek Sowiński <just at ask.me> wrote:
>> This is a continuation of a recent thread "Synchronized const methods"
>> on D.learn.
>>
>> Currently only one thread at a time can execute a synchronized method.
>> But think about D's const -- it's deep, so if a method is const, it
>> can't possibly mutate its object. So many synchronized const method
>> calls could safely look-but-not-touch the same object at the same time.
>>
>> The chain of questions that stems from the above is:
>> 1. Is letting many readers in on an object really safe? Know any
>> multi-threading quirk that would make sh*t hit the fan?
>> 2. If answer to 1. is yes, would there be room in the current
>> implementation of synchronized keyword for a readers-writer lock?
>> 3. If answer to 2. is yes, how do we tackle write-starvation? In a
>> read-mostly scenario the mutating thread may be held up forever.
>>
>> More on readers-writer lock:
>> http://en.wikipedia.org/wiki/Readers-writer_lock
>>
>>
>> Tomek
> 
> This has been suggested before and has been rejected. The major issue is
> that CREW (concurrent-read exclusive-write) locks are known to be not
> composite and therefore its trivial to write code which results in a
> deterministic dead-lock. The problem lies in that the const method can
> have access to a non-const reference to its object via method arguments
> and/or globals. Thus, a read-lock can be obtained first and then later a
> write lock is attempted. Since the first read lock will never be
> released, the write lock can never be taken and a deadlock occurs.

	Unless the write lock can be acquired when the only thread holding
the read lock is the same as the one wanting the write lock.
Something similar is already done for the "standard" lock used by
synchronized methods (otherwise, a synchronized method would be
unable to call another synchronized method without deadlocking).

		Jerome
-- 
mailto:jeberger at free.fr
http://jeberger.free.fr
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