Re: What's an "MPSAFE" driver need to do?

[David Young <dyoung%pobox.com@localhost>]

On Thu, Feb 28, 2013 at 09:43:49AM +0100, Manuel Bouyer wrote:
> On Thu, Feb 28, 2013 at 02:29:11AM -0500, Mouse wrote:
> > [...]
> > Well, assuming rwlock(9) is considered a subset of mutex(9) for the
> > purposes of that sentence, I then have to ask, what else is there?
> > spl(9), the traditional way, specifically calls out that those routines
> > work on only the CPU they're executed on (which is what I'd expect,
> > given what they have traditionally done - but, I gather from the
> > manpage, no longer do).
> > 
> > This then leads me to wonder how a driver can _not_ be MPSAFE, since
> > the old way doesn't/can't actually work and the new way is MPSAFE.
> 
> A driver not marked MPSAFE will be entered (especially
> its interrupt routine, but also from upper layers) with
> kernel_lock held. This is what makes spl(9) still work.
> In order to convert a driver using spl(9)-style calls, you have to replace
> spl(9) calls with a mutex of the equivalent IPL level (a rwlock won't work
> for this as it can't be used in interrupt routines, only thread context).

I want to complicate this idea of spl->mutex conversion a bit.  I used
to think that replacing spl calls by mutex calls would block the same
interrupts that traditionally spl blocks.  Then I realized that I'd
been misled both by the "lore" surrounding spl->mutex conversion, and
by reading (and re-reading) the manual: a mutex initialized with level
`ipl' does NOT necessarily block interrupts.  It will block them if
it is a spin mutex (initialized with one of the hardware interrupt
levels: IPL_VM, IPL_SCHED, IPL_HIGH), but it will not if it is an
adaptive mutex (initialized with one of the software interrupt levels,
IPL_SOFT*).  So things are not 100% symmetrical in mutex land.

Generally you're safe if both your interrupt handlers and your code
running in the a "normal" thread context acquire & release the same
mutex in critical sections.

Dave

-- 
David Young
dyoung%pobox.com@localhost    Urbana, IL    (217) 721-9981