On Mon, Aug 05, 2002 at 10:42:37PM +0800, Edward Shao \(邵治國\) wrote:
> sorry, i found it!
> wake_up_locked(&sem->wait);
> but why do we need to wake up the sleepers again?
> Thank you very much.
>
> -Edward Shao-
>
> ----- Original Message -----
> From: "Edward Shao (邵治國)" <szg90@cs.ccu.edu.tw>
> To: <linux-kernel@vger.kernel.org>
> Sent: Monday, August 05, 2002 10:33 PM
> Subject: a question about __down() in Linux/arch/i386/kernel/semaphore.c
>
>
> > Hi,
> >
> > I have a question about __down() in kernel 2.4.18
> > (Linux/arch/i386/kernel/semaphore.c)
> > I found the last line of __down() is
> > wake_up(&sem->wait);
> > but in kernel 2.5.28, i didn't see this line..
> > is this line necessary in kernel 2.4.18?
> > why?
> >
> > Thank you very much.
> >
> > Best Regard!!!
> >
> > -Edward Shao-
> >
> >
The quick answer: so we don't miss waking someone up. But, seriously,
the semaphore code is very subtle.
This semaphore implementation allows more than one process to be in the
critical section at a time (a.k.a. a counting semaphore). In order to
support those semantics, more than one wakeup may occur before a process
is pulled off the wake_q and changed to running. Because the process that
is waiting to run (in the __down() code) is responsible for pulling itself
off the wait_q, if the 2 __up()s happen before the __down() can finish,
the 2 __up()s will wakeup the same process twice. So, the __down() code
needs to protect agaist this.
-- Bob Miller Email: rem@osdl.org Open Source Development Lab Phone: 503.626.2455 Ext. 17 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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