Hi,
I have a data structure which is used in a kernel thread and in my
ioctl function. I know for interrupts I need to use spin_lock_irqsave
to disable interrupts but what about syscalls (ie. exceptions)?
When the kernel thread accesses the data structure, should it also
disable interrupts so that my user program doesnt send a syscall at
that particular point in time?
thanks
On 7/3/06, mzubair <mzubair173@xxxxxxxxx> wrote:
Thanks Adrian for the reply. So a syscall will not interrupt the local CPU?
If my kernel thread does a spin_lock() to lock the structure and if a
syscall happens right after that will it interrupt the kernel thrread
and try to spin_lock too?
a syscall is an exception (a software interrupt); if you use spin_lock() in your kernel thread you will just be protecting your data structure; the system call (the exception) has nothing to do with that structure, until the system call handler is called (the ioctl routine, in your case); if your ioctl routine uses the same structure than you should protect it using a spinlock also
please note that when a system call is made, first there is an analysis of the system call arguments (registers), then a switch to kernel (supervisor) mode then a search for the proper system call handler routine; this is the exception (software interrupt) handler; as long as it doesn't access the protected structure (and it never does, unless you were mangling with the sys_call_table structure) nothing bad ( a.k.a. inconsistency due to lack of synchronization) will ever happen
when the ioctl routine is finally executed, there is no exception context; you are in kernel mode, in process context; the kernel simply executes the ioctl routine on behalf of the process that requested it through the proper system call; the syncrhonization methods that I have mentioned in the previous mail (spinlock, semaphore, preempt_disable) will work fine here
you should definitely consult chapter 9 of Linux Kernel Development 2nd Edition by Robert Love; it presents all the kernel synchronization methods and their advantages/disadvantages quite clearly
Razvan
P.S.: I apologize to you and to the members of this discussion list for not sending the previous message to everyone (I did a "Reply" instead of "Reply to all" and only you received it)
--
Computers don't make mistakes... What they do they do on purpose!
