Re: [PATCH 0/3] offload bios to a thread

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jul 06 2016 at  9:53am -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Wed, 6 Jul 2016, Mike Snitzer wrote:
> 
> > On Mon, Jul 04 2016 at  6:53pm -0400,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > Hi
> > > 
> > > This is the second version of patches that fix deadlocks by redirecting 
> > > bios from current->bio_list to rescuer workqueues.
> > > 
> > > I found out that the original patches caused deadlock with the loopback 
> > > device. When the loopback device is used, both lower and upper filesystems 
> > > use the same bio set - fs_bio_set. Consequently, bios submitted by both of 
> > > them end up on the same rescuer workqueue. There is a deadlock possibility 
> > > - if generic_make_request for the upper filesystem's bio blocks (because 
> > > there are too many requests in flight on the loop device), it may stall 
> > > processing some bios for the lower filesystem.
> > > 
> > > Ideadlly, each filesystem should have its own bio set. But it doesn't. So 
> > > I fix this problem by not offloading bios allocated from fs_bio_set.
> > 
> > I'd much preferred you just send an incremental fix that built on the
> > tree you know I started, here:
> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip
> 
> You need to change three patches in your git:
> * block: flush queued bios when process blocks to avoid deadlock
> * block: prepare for timed offload of queued bios to workqueue
> * block: use timed offload of queued bios to a workqueue
> because this bug is present in all of them.
> 
> When these patches are sent to Linus, the bug should not be present in any 
> of them.

Yes, I'm aware.  Please review:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip