[armedbear-devel] [Updated4] Patch for ABCL against BORDEAUX-THREADS HEAD

[Mark Evenson]

On 3/18/11 4:41 PM, Ville Voutilainen wrote:
> On 18 March 2011 17:37, Mark Evenson<evenson at panix.com>  wrote:
>> This was the previous implementation, which was the subject of the
>> criticism:
>>
>> (defun condition-wait (condition lock)
>>   (check-type condition condition-variable)
>>   (release-lock lock)
>>   (do ()
>>       ((acquire-lock lock nil))
>>     (threads:synchronized-on condition
>>        (threads:object-wait condition))))
>
> If this is directly mapped to a pthread implementation, the loop must be there.

I think it is slightly different, as the semantics of BT:CONDITION-WAIT 
are merely that the lock is always reacquired before returning.  The 
caller of BT:CONDITION-WAIT is required to check for spurious wakeups.

>
>> I believe the race condition is on the the synchronization on the condition
>> variable, not on the mutex lock.
>
> The condition variable needs to use the mutex lock given, not a separate lock.

The condition variable interface is using the mutex lock specified in 
BT:CONDITION-WAIT.  For the ABCL implementation, I'm using the Java 
monitor associated with the condition variable for the additional 
synchronization needed to implement BT:CONDITION-NOTIFY which couldn't 
use the mutex lock without contention problems.  But definitely check my 
logic here, as it has been faulty before.

-- 
"A screaming comes across the sky.  It has happened before, but there
is nothing to compare to it now."