On 3/19/07, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
>
> I may be wrong, but from what I understand, the count passed in the
> semaphore initialization
>
> sema_init(sem, count);
>
> is the number of threads that are allowed to hold the semaphore
> simultaneously (no of threads that are allowed to enter their critical
> sections simultaneously). So can't a semaphore be actually held
> recursively?
having multiple holders doesn't imply allowing recursion!
just think of this: say you set the count such that 2 holders are
allowed.
your code then does
down(&sem);
....
down(&sem);
and most of the time this will work.
until another instance of your code runs in parallel:
cpu 0 cpu 1
down(&sem);
down(&sem)
...
....
down(&sem); <- dead
down(&sem); <- dead
at which point you're very deadlocked.
Ah ... I got it ... Thanks!
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