Re: semaphore and spin locks

[Ralph Corderoy <ralph@inputplus.demon.co.uk>]

Hi Werner,

> > 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.

When you say `process' do you mean a normal Unix process?  If so, the
kernel will pre-empt it whilst it's in its busy loop and the other,
spin-lock holding, process will get a chance to release it.

Or does `process' mean something running as part of the kernel and not
pre-emptable in this case?


Ralph.

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