On 01/03/2011 22:52, Spacen Jasset wrote:
On 01/03/2011 16:59, d coder wrote:
> I'm afraid that I have no idea what would be "stale" about a shared
variable.
> sychronized uses a mutex, and if you want to avoid race conditions,
you need to
> use mutexes or something similar when dealing with shared variables.
But I don't
> know what would be "stale" about a variable.
>
One thread modifies a shared variable and the other thread still gets an
old value. I do not know if this is applicable to D at all. Just wanted
to get a clarification after I read an article in "Java Concurrency in
Practice" book. I quote a relevant paragraph:
Locking is not just about mutual exclusion; it is also about memory
visibility. To ensure that all threads see the most up-to-date
values of shared mutable variables, the reading and writing must
synchronize on a common lock.
Regards
Perhaps what you mean is synchronising a function vs shared data object.
If you have one Function A and some data D, then so long as ONLY
function A (and it's callees) change or read D, then everything should
work fine. Hence you're synchronising function A (or putting it in a
critical section)
However, if you have two functions A and B, and data D. If function A
and B can read and or write to the data D, then thread 1 can execute in
A (but no other if it's synchronised) But, it may be pre-empted and
another thread can then start executing in function B that is also
synchronised. The problem here is that function A is half way though
execution when B starts, hence you could likely have data in D that has
been partially changed, "old" other otherwise inconsistent, which can
cause problems, or even proper disasters.
The answer in this case is to have a mutex to protect D, so function A
or B must obtain this and hold it while is modifies data D. Any other
function will then have to wait for the mutex to be unlocked before
reading or writing.
See: http://www.digitalmars.com/d/2.0/statement.html#SynchronizedStatement
So you can EITHER use synchronised to mark a block of code for execution
by one thread only.
OR if you provide it with an expression, you mark the block to be run
only if an object is "unlocked", therin only one thread may accesses a
bit shared of data.
AFAIK this very similar to java
http://www.herongyang.com/Java/Synchronization-Support-in-Java-synchronized.html
