locking questions (regarding file systems)

[Eric Anderson]

On 08/02/06 14:34, R. B. Riddick wrote:
> --- Eric Anderson <anderson at centtech.com> wrote:
>> Here's basically what I do:
>> in the mount function for the FS, I do something like this:
>>
>> DROP_GIANT();
>> g_topology_lock();
>> error = g_vfs_open(devvp, &cp, "fsname", 0);
>> g_topology_unlock();
>> PICKUP_GIANT();
>>
>> What is needed in my unmount function to release those locks?  I've
>> tried some combinations of things, like:
>>
>> DROP_GIANT();
>> g_topology_lock();
>> # wedges here
>> g_vfs_close(cp, td);
>> g_topology_unlock();
>> PICKUP_GIANT();
>> vrele(devvp);
>>
> 
> So the first un-mount works fine?
> And the second un-mount wedges _before_ g_vfs_close?
> 
> I cannot find anything really suspicious in ur code...
> 
> Just 2 thoughts:
> 
> 1. Do we really hold GIANT, when we mount and un-mount something?
> 
> 2. R u sure, that we need vrele()? I mean: Why doesn't g_vfs_close() call
> vrele(), if g_vfs_open() increases that use-count variable? Can u print the
> use-count variable in the beginning and the end of the mount/un-mount

Looks like after mounting it, the use_count is 1.  When unmounting, it 
starts at 1, and moves to 0 after doing a vflush, a g_topology_lock, but 
before the g_vfs_close.

Here's the unmount code snippet:

# here use_count is 1
	error = vflush(mp, 1, flags, td);
	if (error)
		return (error);
		
	DROP_GIANT();
	g_topology_lock();
# this is where the use_count is now zero, and it blocks
	g_vfs_close(cp, td);
	g_topology_unlock();
	PICKUP_GIANT();
	vrele(devvp);
	
Is it blocking because the use_count is already 0?  Is the vflush 
breaking things?

Thanks!
Eric



-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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