Re: [PATCH RFD] alternative kobject release wait mechanism

Hello,

Alan Stern wrote:
On Wed, 18 Apr 2007, Tejun Heo wrote:

Hello, all.

Agreed with the problem but I'm not very enthusiastic for adding
kobj->owner. How about the following? exit() routines will have to
do device_unregister_wait() instead of device_unregister(). On return
from it, it's guaranteed that all references to it are dropped and
->release is finished. The caller is responsible for avoiding
deadlock, of course.

There's a problem with this approach.

Many drivers, especially those for hot-pluggable buses, register and
unregister devices dynamically. These events can occur in time-critical
situations, where the driver cannot afford to wait for all the references
to be dropped when unregistering a device. It's okay to wait in a module
exit routine, but to make things work the routine would have to wait for
references to _all_ unregistered objects to go away, not just the
references for the objects it unregisters at exit time.

So let's see what changes are needed to make the approach workable. We
will have to maintain a count of objects whose release methods haven't
been called yet. The count has to be incremented every time an object is
unregistered (or registered, it doesn't matter which) and decremented
_after_ the release method returns -- meaning somewhere in the driver
core. When the count goes to zero, the exit routine is then allowed to
terminate.

Hmmm, this is beginning to sound like a module-wide refcount which serves
to block mod->exit(). In fact, it sounds almost identical to what
Cornelia wrote, except that the refcount refers only to devices rather
than arbitrary kobjects (and except that the blockage just before
mod->exit returns instead of just after). You can see where I'm
leading...

The goal of immediate-disconnect is to remove such lingering reference
counts so that device_unregister() or driver detach puts the last
reference count.

You tell a higher layer that a device is going away, on return from the
function, that layer isn't gonna access the device anymore. ie. On
return from the unregistration function, the upper layer shouldn't have
any reference to the device. If you unregister from all layers a device
is registered to, you are left with only 1 reference which you put with
device_unregister(). After all are converted, reference count doesn't
mean much. struct device isn't a reference counted object anymore.

I don't think this is gonna be too difficult to do. I think I can
convert block layer and IDE/SCSI drivers without too much problem.
Dunno much about other layers tho.

Incidentally, Tejun, I'm all in favor of a immediate-detach driver model
approach. Unfortunately it's impossible to realize fully, although we
could come much closer than we are now.

Here's an example where immediate-detach cannot be implemented. A driver
binds to a device and uses that device is a kernel thread. The thread
carries out certain operations which require it to hold the device
semaphore (because, for example, they need to be mutually exclusive with
unbind).

The driver's remove() method is called with the semaphore held. If the
thread tries to lock the semaphore at the same time and blocks, there is
no way at all for the remove() method to force the thread to drop its
reference.

This isn't merely a theoretical example. The USB hub driver works in
exactly this way.

Dunno if I understood the problem right but can't we do the following?

remove()
{
acquire sem;
device_del();
release sem;
device_put_wait();
}

--
tejun
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Relevant Pages

  • Re: Error Handlers for VB6
    ... > No matter how you exit that routine, VB will drop the reference to that class ... > Private Sub Class_Initialize ...
    (microsoft.public.vb.database.ado)
  • Re: How to clear controls from a form - Help Needed
    ... the black box function that reference is removed helps. ... CallersDR = GetData ... black box routine ) as well? ... There is only one SqlDataReader. ...
    (microsoft.public.dotnet.languages.vb)
  • Re: How should an exit routine wait for release() callbacks?
    ... module's exit routine has to wait until the releaseroutine for that ... The module has now given up any reference to the device it ... reference to it and its release routine gets passed to the driver core. ... subsystem can't be unloaded without unloading my driver first. ...
    (Linux-Kernel)
  • Re: Larry Wall, on Tcl
    ... In Tcl, objects ... that no other object can reference this object without trying ... At this point the GC routine ... failure and make a decision on what needs to be done upon failure. ...
    (comp.lang.tcl)
  • Re: Using with
    ... It was a fair test, both routines use the exact same variable reference, ... still, using your change, I again find using With to be the faster method. ... routine, that was by design. ...
    (microsoft.public.vb.general.discussion)