hash synchronization on linux

Thu Jul 8 22:28:00 GMT 2004

The problem is in thread 8. I doubt it has been fixed,
but I didn't check.
Basically we have
_Jv_MonitorEnter
	acquires lock bit on lightweight lock entry
	notices it needs to allocate heavyweight lock
	calls eventually alloc_heavy.
alloc_heavy call GC_local_gcj_malloc
GC_local_gcj_malloc is out of free list entries AND
	is first allocation to run since GC, which
	results in a call to
FinalizerThread.finalizerReady, which is currently
	unrestricted Java code. It then calls
_Jv_MonitorEnter
	which tries to acquire the same lock table
	entry it already owns ==>
deadlock
Is it hard to make FinalizerThread.finalizerReady a small piece
of C++ code, which does not
acquire Java locks, e.g. by just doing a pthread_cond_signal
equivalent?
The alternative would be to avoid allocation calls with the
lock bit on a lock table entry set. (I think that registering
a finalizer is OK. That allocates internally, but I don't
think it can invoke finalizer notification.) But that looks
a bit messy and more expensive: You'd have to preallocate the
heavyweight object before acquiring the lock bit, and then drop
it if you didn't need it.
Or I guess we could add a GC allocation call which didn't do the
finalizer notification check.
I'm also not sure that running Java code during finalizer
notification is otherwise safe. This can happen during arbitrary
allocations, with locks held, etc.
Hans
> -----Original Message-----
> From: java-owner@gcc.gnu.org 
> [mailto:java-owner@gcc.gnu.org]On Behalf Of
> Jacob Gladish
> Sent: Thursday, July 08, 2004 2:32 PM
> To: java@gcc.gnu.org
> Subject: hash synchronization on linux
>>> I've suspected a problem with the hash synchronization for 
> some time now
> and decided that I might be able to reproduce the problem with a unit
> test, and think that I have stumbled onto something. 
> I've attached two classes that can be run against gcc-3.1 and very
> quickly deadlock. To make this happen you must rebuild with a changed
> hash_entry table size of 2. I suspected a race with bucket 
> access (heavy
> lock chain) so reducing the size increased the likelihood that I would
> be able to reproduce this easily. This is being run on Linux 
> SMP 2.4.18
> (machine has two procs).
> I've also included the backtrace of all the threads in this state. No
> threads ever return from _Jv_MonitorEnter. I know the 3.1 release is
> pretty old at this point, but I'm in the middle of building 3.4.1 with
> the hash_entry table size shrunk to 2 and am going to see if 
> it behaves
> the same. I was also under the impression that the 
> hash_synchronization
> code has basically been untouched for a while.
> Thanks
> Jake
>