Re: What does volatile guarantee?
On 2/12/2010 7:23 AM, Roedy Green wrote:
Originally I thought volatile just suppressed caching a value in a
register, so that other threads going to RAM would see the latest
value.
IIRC at least in some early JVM, you could not count on 64-bit loads
and stores being atomic.
But on reading the JLS, it seems to guarantee even more. It says all
threads must see a consistent value.
This suggests if your hardware stores a 64-bit long in two 32-bit
stores, it must somehow lock the pair together so that another thread
will not see half the store. I wonder just how they do that, and if
they indeed do.
I don't think the JLS guarantees that a volatile x++ must be atomic.
It appears to be silent on the topic, which I take to mean you can't
count on it.
Any thoughts?
From Java Concurrency in Practice by Goetz, "For example the semantics
of volatile are not strong enough to make the increment operation (x++)
atomic, unless you can guarantee that the variable is written only from
a single thread."
The other, and I think most important side effect of volatile is, "The
visibility effects of volatile variables extend beyond the value of the
volatile variable itself. When thread A writes to a volatile variable
and subsequently thread B reads that same variable, the values of ALL
variables that were visible to A prior to writing to the volatile
variable become visible to B after reading the volatile variable" again
from Java Concurrency in Practice.
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Knute Johnson
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