cvs commit: src/sys/dev/md md.c

[Don Lewis]

On 16 Sep, Garrett Wollman wrote:
> <<On Thu, 16 Sep 2004 15:15:15 -0700, Nate Lawson <nate at root.org> said:
> 
>> You should be checking the work condition in thread 2 while holding the 
>> mutex but before going to sleep.  Adding work to the queue happens in 
>> thread 1 where you write "..." and that is done with the mutex held so 
>> there is no race.  The full diagram with this detail included is:
> 
> Of course, getting this right is complicated enough that we have an
> entire abstraction to assist.
> 
>> thread1               thread2
>> -----------------------------
>> mtx_lock(mtx)
>> add work to queue
>> mtx_unlock(mtx)
>>                        mtx_lock(mtx)
>> wakeup(ptr)
>>                        check queue for work item
>>                        if (!work item)
>>                            msleep(ptr, mtx)
>>                        else
>>                            dequeue work item and loop
> 
> mtx_lock(mtx)
> add work to queue
> cv_signal(worktodo)
> mtx_unlock(mtx)
> 			mtx_lock(mtx)
> 			for (;;) {
> 				check queue for work item
> 				if (!work item)
> 					cv_wait(cv, mtx)
> 				else {
> 					dequeue work item
> 					do work
> 				}
> 			}
> 			mtx_unlock(mtx)

It looks to me like there is a race condition in the cv_wait()
implementation.

        			cvp->cv_waiters++;
        			DROP_GIANT();
        			mtx_unlock(mp);
mtx_lock()
...
if (cvp->cv_waiters > 0) {
	cvp->cv_waiters--;
	sleepq_signal();
}
        			sleepq_add(...);
        			sleepq_wait(cvp);


Also, doesn't this potentially have the same problem with extra context
switches that Nate mentioned earlier?