Hello Govind,
> What happens when you go for spin locks without
> disabling kernel preemption? Suppose you acquire
> a spin lock in a system call handler (a service
> routine on behalf of a user mode process) with
> kernel preemption enabled.
This is not possible. Spin lock APIs disable kernel preemption
automatically.
Thanks,
Rajat
________________________________
From: [email protected]
[mailto:[email protected]] On Behalf Of govind raj nayak
Sent: Tuesday, October 06, 2009 3:56 PM
To: [email protected]
Cc: [email protected]
Subject: Re: locking and interrupts
If a data structure is being accessed both by an interrupt
handler and a process / thread / in kernel mode then you might have to
go for spin locks with kernel preemption disabled.
What happens when you go for spin locks without disabling kernel
preemption? Suppose you acquire a spin lock in a system call handler (a
service routine on behalf of a user mode process) with kernel preemption
enabled. When an interrupt handler comes it will preempt the current
process and try to acquire a spinlock. A DEADLOCK!
You can't go for semaphores either since implementation of
semaphore would suspend the process and in an interrupt handler there is
no way to no the context of a process. Though I believe recent kernel
versions do implement semaphores without suspending the process. I guess
you can use these constructs inside interrupt handlers.
To some extent answer to your question also depends on where the
critical section code is?
a) If it's in an exception handler then it's always better to go
for spinlocks with kernel preemption disabled If interrupt handler is
also accessing the same data structure
b) If it's in an interrupt handler then you will have to go for
spin locks with interrupts disabled.
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
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