Re: semaphore and spin locks

[<w.us@verizon.net>]

On Sun, 5 Aug 2001, Thomas Hofer wrote:

> atul wrote (Dienstag, 10. Oktober 2000 03:25):
>
> > Afa i know semaphores provide a very good mechanism for
> > synchronization so why spin locks are needed n how they differ from
> > semaphores..??
>
> I'm just a kernelnewbie, so correct me if I'm wrong:
>
> A spin lock is a primitive mechanism to prevent two or more processes
> accessing a certain data structure at the same time. The lock is an
> integer that's initially set to 0. A process which wants to enter the
> critical area has to set the lock to 1 using an atomic "if 0 then set
> 1"-operation. If the lock is in use, the process has to busy-wait,
> spinning in a tight loop. ==> CPU-time-consuming if you have to wait
> long, but very efficient if you don't have to wait.

Spin locks only work in a multiprocessor environment.
If you use it in a unprocessor environment, then the waiting process would
keep running in a spin and have no chance to release it and the whole
kernel would hang.

Werner


>
> A semaphore on the other hand is a more complicated mechanism consiting
> of a count, a wait-queue and a lock. Processes which wait for a
> semaphore don't do busy waiting, but have to do a context switch, which
> takes some time. So a simple spin lock can be more efficient if you
> expect that there is a good chance that no process has to wait long for
> the lock. But you have to use it with care.
>
> Thomas.
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