When is to preempt safe?

[Chetan Nanda]

On 7 Oct 2011 16:58, "Parmenides" <mobile.parmenides at gmail.com> wrote:
>
> Hi,
>
>   Preemption has two cases: user preemption and kernel preemption. I
> have tow questions about them.
>
> 1. According to Love, "If the kernel is returning to user-space, it
> knows it is in a safe quiescent state. In other words, if it is safe
> to continue executing the current task, it is also safe to pick a new
> task to execute." What's the meaning of user preemption's safety? How
> can we deduce safety of schedule from the current task going on?
>
> 2. Another statement from Love is that "the kernel can preempt a task
> running in the kernel so long as it does not hold a lock". Why is it
> not safe while kernel code holding lock?
New task pick by scheduler may try to get the same lock resulting in
deadlock
>
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