Re: Volatile happens before question
On 1/18/12 11:59 AM, markspace wrote:
On 1/18/2012 11:15 AM, raphfrk@gmail.com wrote:
On Jan 18, 6:12 pm, markspace<-@.> wrote:
Yeah, it's a trick.
You mean yeah, it has the same problem, so won't work either?
Looking at his code a bit more carefully, I think it has problems, but
not the same one. I guess the extra variables in his code that you're
looking at are local variables. So those are safe, they don't figure
into multi-threading concerns.
The bit that confuses me is that he "protects" his map with a write
counter:
writeCounter.getAndIncrement();
map.put(key, value);
writeCounter.getAndIncrement();
And then allows the reader(s) into the critical section whenever the
writeCounter is even (why not just decrement the counter after a write
access?).
Because the reads need to be restarted if there has been any write.
Therefore its not enough to protect with just an "is zero".
while (((save = writeCounter.get()) & 1) == 1);
value = map.get(key);
So if a reader starts "first" and passes the first test
(writeCounter==0), and then starts to access the map, and then a writer
comes in and also begins to access the map... yeah you have two threads
accessing the map at the same time. This doesn't seem to work at all.
That's why he has "save". If the write happens and then the read
attempts to read, the read needs to retry.
(Hint: ConcurrentHashMap
Agreed, if you need this behavior. On the other hand, if you are
experimenting with concurrency code, rather than trying to write
concurrent applications, then this thread is more useful than knowing
about CHM.
<http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html>)