Specifically I am talking about implies() vs add(). I contend that data races
between internal state referred to by and set by (respectively) these two
methods is a non-issue. Namely, the guarantees that "volatile" (which did not
exist as a workable declaration when this code was written) provides are enough
to allow DynamicPolicyProvider.DomainPermissions to have implies() and add()
implementations that look like the following. This is a partial version of this
class. Basically, I removed all use of synchronized (and the assert check in
getPermissions() for Thread.holdsLock(this)) and instead used copy-on-write and
volatile to manage access to the values held in perms and grants.
If add() and implies() are being used concurrently in a data race kind of way,
then even synchronized() doesn't guarantee which version of the data will be
visible at the moment the implies() is called because another thread doing add()
may not be scheduled in a way to guarantee that it calls add() before implies()
is called for example.
private class DomainPermissions {
private volatile PermissionCollection perms;
private volatile List grants = new ArrayList();
DomainPermissions(ProtectionDomain pd) {
Principal[] pra;
principals = (pd != null && (pra = pd.getPrincipals()).length > 0)
? new HashSet(Arrays.asList(pra)) : Collections.EMPTY_SET;
perms = cacheBasePerms ? basePolicy.getPermissions(pd) : null;
}
void add(Permission[] pa) {
List g = new ArrayList(grants);
PermissionCollection pc = new Permissions();
if( perms != null ) {
Enumeration<Permission> e = perms.elements();
while( e.hasMoreElements() ) {
pc.add( e.nextElement() );
}
}
for (int i = 0; i < pa.length; i++) {
Permission p = pa[i];
g.add(p);
if (perms != null) {
pc.add(p);
}
}
grants = g;
if( perms != null )
perms = pc;
}
boolean implies(Permission p, ProtectionDomain domain) {
if (perms != null) {
return perms.implies(p);
}
if (basePolicy.implies(domain, p)) {
return true;
}
if (grants.isEmpty()) {
return false;
}
return getPermissions(false, domain).implies(p);
}
}
Gregg Wonderly
Peter Firmstone wrote:
Ok, You'll have to forgive me, I'm not near the source code at the
moment, those last ideas appear useless, but I'll throw out some more
ideas, I could be wrong.
I think the synchronisation problems stem from the java libraries.
java.security.Policy - abstract class extended by DynamicPolicyProvider,
DynamicPolicyProvider appears to be a wrapper class, it has a useful
constructor:
DynamicPolicyProvider(Policy basepolicy)
The javadoc tends to indicate that you need a new implementation of
Policy that implements the DynamicProvider interface.
You might also want to extend PermissionCollection which contains
Permission objects.
Have a look at Doug Lea's concurrency utilities interest site:
http://gee.cs.oswego.edu/dl/concurrency-interest/
There are plenty of lock free strategies & code you can utilise. If I
get some time, I'll have a look on the weekend.
Cheers,
Peter.
Peter Firmstone wrote:
Peter Firmstone wrote:
Gregg Wonderly wrote:
Peter Firmstone wrote:
Gregg Wonderly wrote:
I have been looking into some seemingly slow responses in several
clients running simultaneously, and I see in some stack traces
that there are synchronization points in
DynamicPolicyProvider.implies() that seem to be heavily
contended. We probably need to revisit this class and rewrite it
to use copy on write mutation so that reads (the majority of
activity) are completely uncontended.
Any thoughts or experience with this issue?
This sounds like a job for
java.util.concurrent.ReentrantReadWriteLock! Da dat, da dat, da
dat, da da! Requires Java 5, works well, the javadoc is clear
too. Can you submit this as an issue on Jira?
We don't actually want to lock, we just want to use a copy on write
update strategy that does lock but set volatile references to the
new contents.
If you have multiple references containing object state, I'd suggest
using an immutable wrapper object (no setters) containing implicit
references to the objects, where it is read or replaced using a single
AtomicReference. The problem you have then is whether the state your
wrapping has visibility elsewhere or not, it is likely that these will
all need to be created by using defensive copies in your constructor,
each time you wish to update state.
In other words you want an AtomicReference, the objects being
de-referenced must be accessed by getting the referent for every
read, it also must not be published (an implicit reference allowed to
escape) after a read. When the AtomicReference is updated it is
guaranteed to be done atomically, however if the referent has
escaped, any escaped (implicit) references will still refer to the
old object. This isn't as easy as it sounds.
Use the compareAndSet() method, in case another write occurs, if the
referent isn't the one expected (it just got updated), you can retry it.
I haven't had time to look into the details so can't comment on
whether this is appropriate or not. You might want to try this and
the ReentrantReadWriteLock and compare performance before deciding.
The contention write lock's cause might be negligible, for code, much
easier to protect, read and understand later on.
Cheers,
Peter.
Gregg Wonderly
