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

[Garrett Wollman]

<<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)


-GAWollman