Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

[Greg KH <greg@kroah.com>]

On Wed, Jan 05, 2011 at 02:51:28PM +0100, Jerome Marchand wrote:
> On 01/04/2011 10:00 PM, Greg KH wrote:
> > On Tue, Jan 04, 2011 at 04:55:13PM +0100, Jerome Marchand wrote:
> >> Also add a refcount to struct hd_struct to keep the partition in
> >> memory as long as users exist. We use kref_test_and_get() to ensure
> >> we don't add a reference to a partition which is going away.
> > 
> > No, don't do this, use a kref correctly and no such function should be
> > needed.
> > 
> >> +	} else {
> >> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> > 
> > That is the function that should properly increment the reference count
> > on the object.
> 
> Agreed.
> 
> >  If the object is "being removed", then it will return
> > NULL and you need to check that.  Do that and you do not need to add:
> 
> The object is actually removed in a rcu callback function. We could
> certainly add a flag to hd_struct, set by the release function, to
> indicate disk_map_sector_rcu() that the partition is being removed, but
> why not use the refcount instead?

Because you have to properly serialize the grabbing of a kref if you
don't have a valid pointer in the first place, otherwise it will not
work properly at all.  Your new function still does not properly handle
the race condition of dropping the last reference and then having the
kref be cleaned up.  You are giving false hope to the user of the api
that what they are doing is correct.

thanks,

greg k-h