svn commit: r240822 - head/sys/geom

[Pawel Jakub Dawidek]

On Wed, Sep 26, 2012 at 01:58:20PM -0600, Kenneth D. Merry wrote:
> On Wed, Sep 26, 2012 at 21:45:41 +0200, Pawel Jakub Dawidek wrote:
> > On Wed, Sep 26, 2012 at 01:21:17PM -0600, Kenneth D. Merry wrote:
> > > On Wed, Sep 26, 2012 at 20:53:39 +0200, Pawel Jakub Dawidek wrote:
> > > > On Wed, Sep 26, 2012 at 11:29:17AM -0600, Kenneth D. Merry wrote:
> > > > > Here is what CAM needs at each step:
> > > > > 
> > > > > 1.  When a device goes away, we need a method to call from daoninvalidate()
> > > > >     (or any other peripheral driver invalidate routine) with these
> > > > >     properties:
> > > > >     - It tells GEOM that the device has gone away, and starts the process
> > > > >       of shutting down the device.  (i.e. withers/orphans the provider)
> > > > >     - It is callable from an interrupt context, with the SIM (MTX_DEF) lock
> > > > >       held, so it can't sleep.
> > > > 
> > > > Neither g_wither_provider() nor g_orphan_provider() require the topology
> > > > lock. They only acquire the event lock, but it is regular mutex, so this
> > > > is fine. Traversing geom's providers list looks like something that does
> > > > need the topology lock, but maybe traversing is not needed at all.
> > > > The reason for this change was a panic in iSCSI initiator where
> > > > disk_gone() was called and provider was destroyed before g_wither_geom()
> > > > returned.
> > > 
> > > Ahh.  How about using LIST_FOREACH_SAFE?  Would that address the problem at
> > > hand?  Are there any other races in there?
> > 
> > It depends. If one geom can hold more than one provider then it might be
> > racy, but from what I see there is always only one provider - there has
> > to be only one, because disk_destroy() destroys it and struct disk
> > represents always only one disk. If that's true then I see not reason to
> > have a loop in there. I'd change it to:
> > 
> > void
> > disk_gone(struct disk *dp)
> > {
> > 	struct g_geom *gp;
> > 	struct g_provider *pp;
> > 
> > 	gp = dp->d_geom;
> > 	if (gp != NULL) {
> > 		pp = LIST_FIRST(&gp->provider);
> > 		if (pp != NULL)
> > 			g_wither_provider(pp, ENXIO);
> > 	}
> > }
> 
> I would suggest doing LIST_FOREACH_SAFE() (with a comment explaining why)
> instead.  That way just in case someone adds another provider down the
> road it will be handled properly.
> 
> Otherwise we need a comment or KASSERT somewhere to explain that we depend
> on there only being one provider, and things will break if there is more
> than one.

I'm happy with adding KASSERT(), as I don't believe LIST_FOREACH_SAFE()
will be safe there, as disk_gone() itself doesn't prevent the function
to be called for two different disks, but one geom. LIST_FOREACH_SAFE()
make it safe to remove only the current element. If any other elements
is removed while we are traversing the list it will most likely panic.

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://tupytaj.pl
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