Messages in this thread | | | Date | Mon, 5 Aug 2002 09:37:43 -0700 | From | Bob Miller <> | Subject | Re: a question about __down() in Linux/arch/i386/kernel/semaphore.c |
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On Mon, Aug 05, 2002 at 10:42:37PM +0800, Edward Shao \(ªòªv°ê\) 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 (ªòªv°ê)" <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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