Hello, Linus. On 08/01/2010 08:01 PM, Linus Torvalds wrote: > This has a proposed patch. I don't know what the status of it is, though. Jens? > > http://marc.info/?l=linux-kernel&m=127950018204029&w=2 > >> Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=16393 >> Subject : kernel BUG at fs/block_dev.c:765! >> Submitter : Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> >> Date : 2010-07-14 13:52 (19 days old) >> Message-ID : <20100714135217.GA1797@xxxxxxxxxxxxx> >> References : http://marc.info/?l=linux-kernel&m=127911564213748&w=2 > > This one is interesting. And I think I perhaps see where it's coming from. > > bd_start_claiming() (through bd_prepare_to_claim()) has two separate > success cases: either there was no holder (bd_claiming is NULL) or the > new holder was already claiming it (bd_claiming == holder). > > Note in particular the case of the holder _already_ holding it. What happens is: > > - bd_start_claiming() succeeds because we had _already_ claimed it > with the same holder > > - then some error happens, and we call bd_abort_claiming(), which > does whole->bd_claiming = NULL; > > - the original holder thinks it still holds the bd, but it has been released! > > - a new claimer comes in, and succeeds because bd_claiming is now NULL. > > - we now have two "owners" of the bd, but bd_claiming only points to > the second one. > > I think bd_start_claiming() needs to do some kind of refcount for the > nested holder case, and bd_abort_claiming() needs to decrement the > refcount and only clear the bd_claiming field when it goes down to > zero. > > I dunno. Maybe there's something else going on, but it does look > suspicious, and the above would explain the BUG_ON(). Yeah, that definitely sounds plausible. I think the condition check in bd_prepare_to_claim() should have been "if (whole->bd_claiming)" instead of "if (whole->bd_claiming && whole->bd_claiming != holder)". It doesn't make much sense to allow multiple parallel claiming operations anyway and the comment above already says - "This function fails if @bdev is already claimed by another holder and waits if another claiming is in progress." I'll try to build a test case and verify it. Thank you. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
