Re: [pool] synchronization issues in Pool
Hi Oliver, The tool is still a research prototype but I intend to release it in the near future. I took a quick look and it seems like my tool might not be very useful on the commons transaction code because that code uses sophisticated locking primitives which the current version of my tool does not handle and hence will produce lots of false alarms (though we have plans to extend it to handle those primitives). Nevertheless, I will run my tool on that code in the near future and get back to you if I find any issues. Best, -- Mayur On 10/23/05, Oliver Zeigermann [EMAIL PROTECTED] wrote: Hi Mayur, that really sounds like a cool tool. You results are also quite impressive. I actually can not comment on them, but would like to know if your tool is available anywhere? If not - and I suppose so - could you also run it on the commons transaction code where any race condition really would be very harmful. I already know of one that has been fixed in CVS, but not in a final release. Thanks in advance Oliver - To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
[pool] synchronization issues in Pool
Hello,
I'm a PhD student in Computer Science at Stanford University,
evaluating a static race detection tool I'm developing on open source
Java programs. I ran it on the 5 implementations of pools in Apache
Commons Pool, and found some bugs. The output of the tool is here:
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/stack/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/softref/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic_keyed/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/stack_keyed/index.html
I will explain one race in detail. Following the [grouped by field]
link on the first link above and then the [details] link under the
heading Races on field org.apache.commons.pool.BaseObjectPool:
boolean closed leads you to:
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic/R8_trace.html
Traversing the [up] links on this page yields at least one pair of
paths in the call graph along which a common lock is not held and
hence a race can occur if that pair of paths is executed by different
threads:
PATH 1:
GenericHarness:27
org.apache.commons.pool.impl.GenericObjectPool:853
org.apache.commons.pool.BaseObjectPool:77
leading to a read access of the field 'closed' at
org.apache.commons.pool.BaseObjectPool:73
PATH 2:
GenericHarness:33
org.apache.commons.pool.impl.GenericObjectPool:901
leading to a write access of the field 'closed' at
org.apache.commons.pool.BaseObjectPool:62
[GenericHarness is a test harness that my tool automatically generates.]
The above race is due to missing synchronization in method
returnObject of class GenericObjectPool. In fact, it can cause a null
pointer exception:
thread 1 calls the returnObject method which executes the assertOpen
method and finds the pool to be open.
thread 2 calls the close method which sets _factory to null.
thread 1 executes the addObjectToPool method which deferences _factory.
Many other races are because parts of some methods implementing the
pool interfaces (ex. invalidateObject below) that access _factory are
not synchronized:
public void invalidateObject(Object obj) throws Exception {
assertOpen();
try {
_factory.destroyObject(obj);
}
finally {
synchronized(this) {
_numActive--;
notifyAll();
}
}
}
Perhaps, the developer expects the client methods implementing the
factory interface to be synchronized. If this is indeed the case, it
should perhaps be documented somewhere so that unwitting users don't
implement the factory interface without synchronization. Even if
users implement the factory interface with synchronization, there is a
problem: if one thread executes the invalidateObject method and
another executes the close method (which sets _factory to null), then
there can be a null pointer exception similar to the scenario outlined
above.
Most of the races I found are harmful (ex. causing null pointer
exceptions) if the close method is called concurrently with some other
pool interface method. Perhaps, it is highly unlikely that a client
would do that. However, since the pool interface is intended to be
thread-safe, I think it's implementations should handle this scenario
gracefully.
I have summarized below all possible bugs I found by inspecting the
above reports generated by my race detection tool. I would be happy
if you can confirm/refute these bugs.
Thanks!
-- Mayur
org.apache.commons.pool.impl.GenericObjectPool
==
The returnObject(Object) and addObject() methods must be synchronized.
The synchronization inside the body of the method addObjectToPool is
then unnecessary since all callers of this method are synchronized.
The entire invalidateObject(Object) method must be synchronized, not
just parts of it.
The entire borrowObject() method must be synchronized, not just parts
of it. Note that there are dereferences of _factory in the
unsynchronized portions of this method, so there will be a null
pointer dereference if it is called concurrently with the close
method. Once the entire borrowObject method is synchronized, you can
also move the assertOpen() call at the start of each iteration of the
loop in this method to outside the loop.
org.apache.commons.pool.impl.StackObjectPool
The entire returnObject(Object) method must be synchronized, not just
parts of it.
The getNumIdle() method must be synchronized.
The getNumActive() method must be synchronized.
The entire addObject() method must be synchronized, not just parts of it.
org.apache.commons.pool.impl.SoftReferenceObjectPool
The entire returnObject(Object) method must be synchronized, not just
parts of it.
The
[pool] synchronization issues in Pool
Hello,
I'm a PhD student in Computer Science at Stanford University, evaluating a
static race detection tool I'm developing on open source Java programs.
I ran it on the 5 implementations of pools in Apache Commons Pool, and
found some bugs. The output of the tool is here:
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/stack/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/softref/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic_keyed/index.html
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/stack_keyed/index.html
I will explain one race in detail. Following the [grouped by field] link
on the first link above and then the [details] link under the heading
Races on field org.apache.commons.pool.BaseObjectPool: boolean closed
leads you to:
http://suif.stanford.edu/~mhn/commons-pool-1.2/build/classes/PASS1/generic/R8_trace.html
Traversing the [up] links on this page yields at least one pair of paths
in the call graph along which a common lock is not held and hence a race
can occur if that pair of paths is executed by different threads:
PATH 1:
GenericHarness:27
org.apache.commons.pool.impl.GenericObjectPool:853
org.apache.commons.pool.BaseObjectPool:77
leading to a read access of the field 'closed' at
org.apache.commons.pool.BaseObjectPool:73
PATH 2:
GenericHarness:33
org.apache.commons.pool.impl.GenericObjectPool:901
leading to a write access of the field 'closed' at
org.apache.commons.pool.BaseObjectPool:62
[GenericHarness is a test harness that my tool automatically generates.]
The above race is due to missing synchronization in method returnObject of
class GenericObjectPool. In fact, it can cause a null pointer exception:
thread 1 calls the returnObject method which executes the assertOpen
method and finds the pool to be open.
thread 2 calls the close method which sets _factory to null.
thread 1 executes the addObjectToPool method which deferences _factory.
Many other races are because parts of some methods implementing the pool
interfaces (ex. invalidateObject below) that access _factory are not
synchronized:
public void invalidateObject(Object obj) throws Exception {
assertOpen();
try {
_factory.destroyObject(obj);
}
finally {
synchronized(this) {
_numActive--;
notifyAll();
}
}
}
Perhaps, the developer expects the client methods implementing the factory
interface to be synchronized. If this is indeed the case, it should
perhaps be documented somewhere so that unwitting users don't implement
the factory interface without synchronization. Even if users implement
the factory interface with synchronization, there is a problem: if one
thread executes the invalidateObject method and another executes the close
method (which sets _factory to null), then there can be a null pointer
exception similar to the scenario outlined above.
Most of the races I found are harmful (ex. causing null pointer
exceptions) if the close method is called concurrently with some other
pool interface method. Perhaps, it is highly unlikely that a client would
do that. However, since the pool interface is intended to be thread-safe,
I think it's implementations should handle this scenario gracefully.
I have summarized below all possible bugs I found by inspecting the above
reports generated by my race detection tool. I would be happy if you can
confirm/refute these bugs.
Thanks!
-- Mayur
org.apache.commons.pool.impl.GenericObjectPool
==
The returnObject(Object) and addObject() methods must be synchronized.
The synchronization inside the body of the method addObjectToPool is then
unnecessary since all callers of this method are synchronized.
The entire invalidateObject(Object) method must be synchronized, not just
parts of it.
The entire borrowObject() method must be synchronized, not just parts of
it. Note that there are dereferences of _factory in the unsynchronized
portions of this method, so there will be a null pointer dereference if it
is called concurrently with the close method. Once the entire
borrowObject method is synchronized, you can also move the assertOpen()
call at the start of each iteration of the loop in this method to outside
the loop.
org.apache.commons.pool.impl.StackObjectPool
The entire returnObject(Object) method must be synchronized, not just
parts of it.
The getNumIdle() method must be synchronized.
The getNumActive() method must be synchronized.
The entire addObject() method must be synchronized, not just parts of it.
org.apache.commons.pool.impl.SoftReferenceObjectPool
The entire returnObject(Object) method
