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@trippelsdorf.de Date : 2010-07-14 13:52 (19 days old) Message-ID : 20100714135217.GA1797@arch.tripp.de 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.