On Wed, 26 Sep 2007, Tejun Heo wrote:

> >> Hmmm... I might be missing something here. Who else can wake up a
> >> thread in uninterruptible sleep?

> >
> > In principle, anything can. There has never been any guarantee in the
> > kernel that a task sleeping on a waitqueue will remain asleep until
> > the waitqueue is signalled. That's part of the reason why things like
> > __wait_event() are coded as loops.

> Hmmm... I always thought the queue was because the condition can change
> inbetween waking up and actually running. For example, if the condition
> is !(queue empty), another task can enter the critical section and
> consume the element which triggered wake up before the woken up task do.

That's the other part of the reason for using a loop. :-)

Alan Stern

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