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

[Kent Overstreet]

On Mon, Sep 03, 2012 at 04:41:37PM -0400, Mikulas Patocka wrote:
> ... or another possibility - start a timer when something is put to 
> current->bio_list and use that timer to pop entries off current->bio_list 
> and submit them to a workqueue. The timer can be cpu-local so only 
> interrupt masking is required to synchronize against the timer.
> 
> This would normally run just like the current kernel and in case of 
> deadlock, the timer would kick in and resolve the deadlock.

Ugh. That's a _terrible_ idea.

Remember the old plugging code? You ever have to debug performance
issues caused by it?

> 
> > I could be convinced, but right now I prefer my solution.
> 
> It fixes bio allocation problem, but not other similar mempool problems in 
> dm and md.

I looked a bit more, and actually I think the rest of the problem is
pretty limited in scope - most of those mempool allocations are per
request, not per split.

I'm willing to put some time into converting dm/md over to bioset's
front_pad. I'm having to learn the code for the immutable biovec work,
anyways.