Hubertus Franke wrote:
> Again, the trick is to not
> sync the state of the kernel and the user level. It comes naturally
> if you properly separate the duties.
>
Your code for usema_down is simply:
int usema_down(ulock_t *ulock)
{
if (!__ulock_down(ulock)) {
return 0;
}
return ulock_wait(ulock,0);
}
This means you do not recheck if the usema is really available after you
return form ulock_wait(), but you may have the following situtation:
Process 1 Process 2 Process 3
down
down
-> call usema_wait
but gets preempted shortly
before it
can enter kernel mode
up
(kernel sema is free)
down -> got it
resume execution,
thinks its also got it
because kernel sema is free!
Now you have two owners!!
Martin Wirth
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This archive was generated by hypermail 2b29 : Thu Feb 28 2002 - 21:00:38 EST