Re: devctl (alike?) for devfs

On Tue, May 13, 2008 at 10:04:56PM +0300, Andriy Gapon wrote:
on 12/05/2008 00:48 Kostik Belousov said the following:
No, we do not have a leak, but we have somewhat non-obvious behaviour.

The cdev structure is freed only after the last reference to cdev is
gone. Typical holder of the reference is the devfs vnode. In the normal
usage, the vnode is present until both the device is destroyed _and_
devfs_populate_loop() run is performed. This function actually reclaim
the vnodes for destroyed devices, that causes last reference to cdev to
be dropped and memory freed.

The populate loop is called syncronously from the upper levels. The
easiest method to trigger it is to do ls /dev, since it is called from
the devfs_lookupx().

Thank you for the explanation! ls did trigger DESTROY notifications.
But this arbitrary delay between device entry going away and
notification about about it is a little bit "uncool".

So I re-wrote the patch to post notifications about deleted devices with
si_refcount > 0 in a fashion similar to how si_refcount=0 devices are
freed. I put those cdev-s onto a special list (re-/ab-using si_siblings
LIST link field) and bump their si_refcount to prevent them from getting
destroyed before the notification is sent (it would need si_name).
Then, in dev_unlock_and_free() I send notifications and call dev_rel on
the devices.

I am not entirely satisfied with the code:
1. I don't like the way I move LIST elements from one head to the other,
this should be a macro in queue.h
2. I am not sure about the names I picked
3. I slightly don't like a fact that parent-child destroy notifications
are sent in reverse order because of my simplistic LIST usage.

I looked at your previous patch, and it seems it is much simpler to
do drop the devmtx once more then to try to abuse free lists.
In the destroy_devl(), after the

while (dev->si_threadcount != 0) {
/* Use unique dummy wait ident */
msleep(&csw, &devmtx, PRIBIO, "devdrn", hz / 10);
}

loop, add

mtx_unlock(&devmtx);
if (!cold)
devctl_notify("DEVFS", dev->si_name, "DESTROY", NULL);
mtx_lock(&devmtx);

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