I'll commit an example implementation shortly called ConcurrentPermissions
Comments Inline below:
Gregg Wonderly wrote:
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();
Looking at the source in java.security.permissions.java , the
perms.elements() method is a synchronized access to a HashMap that has
its elements returned in an iterator by two method calls,
permsMap.values().iterator() , this is still a synchronised access so
you'll not gain any performance advantage, but you'll be copying the
hashmap unneccessarily.
while( e.hasMoreElements() ) {
pc.add( e.nextElement() );
}
There are synchronized accesses occurring here in underlying
implementations, however this operation isn't atomic, writes can occur
concurrently, one or more Permission objects might go missing if writes
occur concurrently to different PermissionCollection objects, where one
replaces the other.
}
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) {
There is still synchronized access in the underlying Permissions object,
no increase in concurrency has been gained due to it's use of
synchronized access of HashMap.
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
