Why can not processes switch in atomic context?

[Rajesh S R]

On Jul 3, 2012 7:55 PM, "Parmenides" <mobile.parmenides at gmail.com> wrote:
>
> Hi,
>
>     It is said that kernel can not be preempted in interrupt context
> and when it is in a critical section protected by a spin lock.
>
> 1. For the spinlock case, it is easy to get if preemption is allowed
> in critical section, the purpose of protection provided by spinlock
> can not be achieved readily.
>
I don't know about Linux kernel. But the very prime purpose of a spin lock
is to synchronize in the presence of pre-emption. Are u talking about
implementing spin lock in the absence of an atomic hardware instruction?

> 2. For the interrupt context case, I think when processing interrupt,
> kernel can be preempted in principle. But, this really increases the
> interrupt processing time which further cause longer response time and
> data missing in device. Except that, is there any other reasons?
>
> 3. Kernel is responsible for prohibiiting passive process switches,
> namely preemption, in the above cases. But, It seems that it does not
> take care of active process swtiches, namely yield. For example, some
> code in a critical section protected by a spinlock can invoke
> schedule() to switch process passively. Is this the case?
>
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