Messages in this thread | | | Date | Wed, 5 Jan 2011 08:27:09 -0800 | From | Greg KH <> | Subject | Re: [PATCH 2/2] block: fix accounting bug on cross partition merges |
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On Wed, Jan 05, 2011 at 05:19:19PM +0100, Jerome Marchand wrote: > On 01/05/2011 05:00 PM, Greg KH wrote: > > 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. > > > > For clarification, is your objection only about not adding that misleading > function to kref api (I understand that), or is my code actually racy?
As you are adding a misleading function to the kref api, and by using it, causing a racy implementation, I would say both :)
thanks,
greg k-h
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