FreeBSD spinlock - compatibility layer

[John Baldwin]

On Wednesday, May 22, 2013 9:27:16 am Alfred Perlstein wrote:
> On 5/22/13 9:05 AM, John Baldwin wrote:
> > Probably not.  For example, on FreeBSD you want your driver lock to be
> > preempted by an interrupt to avoid higher interrupt latency for filter
> > handlers.  Most drivers should not need temporary pinning.  If they want to
> > pin work to threads they should bind threads or IRQs to specific CPUs, not
> > rely on temporary pinning.
> >
> I know how it works in FreeBSD.
> 
> I think that a compatibility layer should first and foremost aim for 
> compatibility, not speed at expense of expected semantics.

The problem with this is that whatever code runs under this layer also has to
cooperate with the rest of the system.  Blindly using spin locks does not do
that.  Also, I think my entire point is about "expected semantics".  People
should think about the actual semantics they need in a driver, not just assume
that whatever side effects they get from the primitives and APIs provided on
one platform defines the semantics they need.  I still assert that in terms of
what a device driver actually expects, a regular mutex will provide the correct
semantics.

-- 
John Baldwin