Learning Linux wrote:
I have a very basic doubt here ... what makes it impossible to sleep in an ISR? I mean, I know that the kernel preemption is disabled and the kernel will panic, but I could not understand why?
The impossibility to sleep comes as a natural consequence of the fact that ISRs have to be fast.
An ISR has to be fast in order to allow other system tasks to happen. Otherwise it will do a lot of work (like busy waiting) and any other interrupts will fail to get handled. That would completely render the system unusable.
On the other hand, when you sleep in a given context, the scheduler is invoked. That means that the ISR routine would be replaced by a process from the scheduler's queue(s). But how would you ever know to reschedule the ISR? After all, it is asynchronous.
This is why ISRs (top halves) and tasklets/softirqs (bottom halves) are said to be running in interrupt context; processes are said to be running in process context.
Interrupt context has very strict rules like being unable to sleep or do a large amount of work (this will render the system unable and, quite likely, a panic would occur).
Hope this helps. You should be read Robert Love's excellent "Linux Kernel Development 2nd Edition". It is quite readable and has a pleasant way of showing you the basic kernel concepts. Chapter 6 is concerned with interrupts and interrupt handling routines (ISRs).
Razvan -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
