A little question about SMP synchronization

queuebil · 09-30-2006, 06:36 PM

Hi all!
I need to synchronize some kernel routines in accessing a shared variable. I would use semaphores (like sema_init(), down_interruptible(), up(), etc.). In order to make it work also on SMP architectures, should I use spinlocks too? Or are semaphores sufficient?
Thank you and sorry for bad english

paulsm4 · 09-30-2006, 09:27 PM

Hi -

The short answer is that, with current versions of Linux (certainly 2.6.x) you can safely use either kernel spinlocks or kernel semaphores with a multi-CPU (or multi-core) SMP system. the same restrictions now apply for SMP as for uniprocessors: if you cannot block, then you need a spinlock. Otherwise, you can (and usually should) use kernel semaphores.

Much more detailed info can be found in this excellent article from Linux Magazine:

Concurrency in the Kernel, Linux Magazine, November 2005
http://www.linux-mag.com/content/view/2360/0/1/6/

'Hope that helps .. PSM

queuebil · 10-01-2006, 04:59 AM

Thank you for aswering, it was very helpful. The article covers almost everything I need to know for now, but...

Quote:

with current versions of Linux (certainly 2.6.x) you can safely use either kernel spinlocks or kernel semaphores with a multi-CPU (or multi-core) SMP system. the same restrictions now apply for SMP as for uniprocessors: if you cannot block, then you need a spinlock. Otherwise, you can (and usually should) use kernel semaphores.

I forgot to say i'm not working on kernel 2.6.x, but on 2.4.25. Is it still safe to use only semaphores with this old version?