Re: [Qemu-devel] [PATCH] block: Make op blocker recursive

[Fam Zheng]

On Sat, 06/21 12:45, Benoît Canet wrote:
> The Saturday 21 Jun 2014 à 16:53:58 (+0800), Fam Zheng wrote :
> > On Fri, 06/20 09:30, Eric Blake wrote:
> > > On 06/19/2014 11:01 PM, Fam Zheng wrote:
> > > > On Thu, 06/19 22:20, Benoît Canet wrote:
> > > >> The Thursday 19 Jun 2014 à 14:13:20 (-0600), Eric Blake wrote :
> > > >>> On 06/19/2014 02:01 PM, Benoît Canet wrote:
> > > >>>> As the code will start to operate on arbitratry nodes we need the op 
> > > >>>> blocker
> > > >>>
> > > >>> s/arbitratry/arbitrary/
> > > >>>
> > > >>>> to recursively block or unblock whole BDS subtrees.
> > > > 
> > > > I don't get the reason, can you elaborate?
> > > 
> > > Consider what happens if I have:
> > > 
> > > base <- snap1 <- active
> > > 
> > > then I start a fleecing NBD server on the state as it was at snap1:
> > > 
> > > base <- snap1 <- active
> > >               \- fleecing
> > > 
> > > then I do a blockpull into active:
> > > 
> > > base <- snap1 <- fleecing
> > > active
> > > 
> > > at this point, base and snap1 are no longer tied to active, but they
> > > STILL must be protected from operations that would modify their contents
> > > in a way that would break the fleecing operation.  The solution we are
> > > looking at is making BDS blockers recursive to every element of the
> > > chain, not just the top-level device.
> > 
> > This would already have been protected by backing blocker of fleecing 
> > target.
> > 
> > > 
> > > Another example: consider:
> > > 
> > > base <- snap1 <- active
> > > 
> > > then someone uses Jeff's proposed new change-backing-file QMP command to
> > > rewrite the snap1 metadata to point to base via a relative name instead
> > > of an absolute name.  It shouldn't matter whether active is blocked, but
> > > only whether snap1 is blocked.  But to know if snap1 is blocked, we have
> > > to propagate the blockers of active down recursively to its backing files.
> > 
> > Why do we need to block changging of metadata? I think this operation is 
> > safe
> > in most cases.
> > 
> > Correct me if I'm missing anything, but even if snap1 _is_ blocked, it 
> > would be
> > because snap1 is serving as backing of active. In this case, the actual 
> > blocker
> > should be active->backing_blocker.
> > 
> > > 
> > > >> What would be a cleaner solution ?
> > > > 
> > > > What is the question to solve?
> > > 
> > > I think Jeff's idea is on target - rather than blocking by operation, we
> > > should instead be blocking on access patterns (various operations
> > > trigger several access patterns):
> > > https://lists.gnu.org/archive/html/qemu-devel/2014-06/msg04752.html
> > > 
> > > Jeff's initial list included:
> > > 
> > > > So if I think of operations that are done on block devices from a
> > > > block job, and chuck them into categories, I think we have:
> > > > 
> > > > 1) Read of guest-visible data
> > > > 2) Write of guest-visible data
> > > > 3) Read of host-visible data (e.g. image file metadata)
> > > > 4) Write of host-visible data (e.g. image file metadata, such as
> > > > the backing-file)
> > > > 5) Block chain manipulations (e.g. movement of a BDS, change to r/w
> > > > instead of r/o, etc..)
> > > > 6) I/O attribute changes (e.g. throttling, etc..)
> > 
> > Most operations looks safe to me, given the way how IOThreads and coroutine
> > work now. It's only the chain manpulations in long running block jobs that 
> > are
> > exclusive, and by nature it should be checked per chain.  Can we set some op
> > blockers on the bottom BDS and check it each time, to prevent user from
> > starting a second chain manipulator?
> 
> I don't know if bottom BDS locking is any good because some driver like quorum
> have multiple childs.
> Locking everytime the root (top) of the tree seems a feasible solution indeed.

Quorom doesn't change the convensions of backing chain, so each child belongs
to its own backing chain, and that chain has a deterministic top and bottom.

Blocking flag on bottom saves us from adding reverse looking up (->overlay
pointer), because we already have the ->backing_hd pointer in BDS.

Fam