Re: [dm-devel] [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers

[Kent Overstreet <koverstreet google com>]

On Tue, Aug 28, 2012 at 06:06:10PM -0400, Vivek Goyal wrote:
> On Tue, Aug 28, 2012 at 10:37:36AM -0700, Kent Overstreet wrote:
> > Previously, if we ever try to allocate more than once from the same bio
> > set while running under generic_make_request() (i.e. a stacking block
> > driver), we risk deadlock.
> > 
> > This is because of the code in generic_make_request() that converts
> > recursion to iteration; any bios we submit won't actually be submitted
> > (so they can complete and eventually be freed) until after we return -
> > this means if we allocate a second bio, we're blocking the first one
> > from ever being freed.
> > 
> > Thus if enough threads call into a stacking block driver at the same
> > time with bios that need multiple splits, and the bio_set's reserve gets
> > used up, we deadlock.
> 
> Hi Kent,
> 
> So above deadlock possibility arises only if multiple threads are doing
> multiple splits from same pool and all the threads get blocked on mempool
> and don't return from ->make_request function.
> 
> Is there any existing driver in the tree which can cause this deadlock or
> it becomes a possibility only when splitting and bcache code shows up?

It is emphatically possible with dm, though you would probably need a
pathalogical configuration of your targets for it to be possible in
practice - at least with the targets currently commonly in use.

With most of the newer dm targets coming down the pipe I expect it
should be possible under real world configurations, with the caveat that
under that kind of memory pressure in practice many other things will
fall over first.