svn commit: r192535 - head/sys/kern

[John Baldwin]

On Friday 22 May 2009 11:37:38 am Scott Long wrote:
> John Baldwin wrote:
> > On Friday 22 May 2009 9:44:18 am Scott Long wrote:
> >> John Baldwin wrote:
> >>> On Thursday 21 May 2009 6:11:02 pm Attilio Rao wrote:
> >>>> At this point I wonder what's the purpose of maintaining the sleeping
> >>>> version for such functions?
> >>> Actually, I still very much do not like using M_NOWAIT needlessly.  I 
would 
> >>> much rather the solution for make_dev() be that the 1 or 2 places that 
need 
> >>> to do it with a mutex held instead queue a task to do the actual 
make_dev() 
> >>> in a taskqueue when no locks are held.  This is basically what 
> >>> destroy_dev_sched() is doing.  Perhaps a make_dev_sched() with a similar 
> >>> callback to be called on completion would be better.  Having a device 
driver 
> >>> do all the work to setup the hardware only to fail to create a node 
in /dev 
> >>> so that userland can actually use it is pretty rediculous and useless.
> >>>
> >> It's a lot easier for me to handle a failure of make_dev in CAM than it 
> >> is to decouple the call to it.  Please don't dictate policy.
> > 
> > But what is there for CAM to handle?  I would expect CAM to handle 
hardware
> > events such as the devices arriving or leaving.  A temporary memory 
shortage
> > it not a hardware event.  As a user, if I insert a USB stick when the 
system
> > happens to be temporarily low on memory, is it more useful for the cdev to
> > appear a few microseconds later from a deferred context once memory is
> > available or for no device to ever appear at all?
> > 
> 
> John,
> 
> You yourself have been recently burned by not fully understanding the
> complexity involved in CAM.  By changing all of the periph drivers to
> conform to an artificial policy limitation of the make_dev call, I face
> a significant amount of time and effort to rewrite and test code paths
> that are, unfortunately, highly complex and very fragile.  Please just
> make a simple concession.

Are you referring to the sysctl thing?  That was quite trivial to fix FWIW.  I 
also do not see why make_dev_sched() with a callback won't work?  We already 
have this exact policy limitation in many similar APIs such as if_attach().  
Another thing to consider is that if you hold a lock while calling into other 
subsystems, that can result in your lock being held for a relatively "long" 
time which increases the chances for lock contention.

-- 
John Baldwin