Jeff Mahoney <jeffm@xxxxxxxx> writes:
> The extent_state structure is used at the core of the extent i/o code
> for managing flags, locking, etc. It requires allocations deep in the
> write code and if failures occur they are difficult to recover from.
>
> We avoid most of the failures by using a mempool, which can sleep when
> required, to honor the allocations. This allows future patches to convert
> most of the {set,clear,convert}_extent_bit and derivatives to return
> void.
Is this really safe?
iirc if there's any operation that needs multiple mempool objects you
can get into the following ABBA style deadlock:
thread 1 thread 2
get object A from pool 1
get object C from pool 2
get object B from pool 2
pool 2 full -> block
get object D from pool 2
pool1 full -> block
Now for thread 1 to make progress it needs thread 2 to free its
object first, but that needs thread 1 to free its object also
first, which is a deadlock.
It would still work if there are other users which eventually
make progress, but if you're really unlucky either pool 1 or 2
is complete used by threads doing a multiple object operation.
So you got a nasty rare deadlock ...
The only way to handle this would be to always preallocate
all objects needed for a operation in an atomic way.
Or never use more than one object.
-Andi
--
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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