If a data structure is being accessed both by an interrupt handler and a process / thread / critical region then you might have to go for a spin lock with kernel preemption disabled if you want to access a datastructure (For instance you could access the datastructure in a system call service routine).
Now, what happens when you try to access the spin lock without kernel preemption? Suppose you have already acquired a spin lock on a data structure in an exception handler (or perhaps in one of the system call handler routine executing on behalf of a process) without disabling kernel preemption. When an interrupt comes then you would end up preempting the process/thread which was executing a system call service routine on behalf of user mode process and try to acquire a spin lock. A DEADLOCK! You can't use semaphore either since implementation of semaphore supspends the process and there is no way to no the context of the process in interrupt handler. Though I believe recent kernel versions do have semaphore implementation which don't suspend the process. I hope this helps.Thanks. -Govindraj On Tue, 06 Oct 2009 12:44:51 +0530 wrote >Dear All,I have a very basic confusion, please help and confirm the right >answer :If a process/thread (user space/kernel space) has taken a lock on a >critical section code, and suddenly an interrupt occurs which want to use the >same shared data of critical region. Will it able to preempt this code which >is running in process context ? As per my understanding, although interrupts has higher priority than process, but it can't preempt the process otherwise a major bug can occur ( depending upon the shared data of critical section). Please confirm my understanding weather its true or not ? Best regards,Krishna
