Re: [PATCH 1/2] dm-snapshot: fix crash with the realtime kernel

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Nov 11 2019 at 11:37am -0500,
Nikos Tsironis <ntsironis@arrikto.com> wrote:

> On 11/11/19 3:59 PM, Mikulas Patocka wrote:
> > Snapshot doesn't work with realtime kernels since the commit f79ae415b64c.
> > hlist_bl is implemented as a raw spinlock and the code takes two non-raw
> > spinlocks while holding hlist_bl (non-raw spinlocks are blocking mutexes
> > in the realtime kernel, so they couldn't be taken inside a raw spinlock).
> > 
> > This patch fixes the problem by using non-raw spinlock
> > exception_table_lock instead of the hlist_bl lock.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > Fixes: f79ae415b64c ("dm snapshot: Make exception tables scalable")
> > 
> 
> Hi Mikulas,
> 
> I wasn't aware that hlist_bl is implemented as a raw spinlock in the
> real time kernel. I would expect it to be a standard non-raw spinlock,
> so everything works as expected. But, after digging further in the real
> time tree, I found commit ad7675b15fd87f1 ("list_bl: Make list head
> locking RT safe") which suggests that such a conversion would break
> other parts of the kernel.

Right, the proper fix is to update list_bl to work on realtime (which I
assume the referenced commit does).  I do not want to take this
dm-snapshot specific workaround that open-codes what should be done
within hlist_{bl_lock,unlock}, etc.

I'm not yet sure which realtime mailing list and/or maintainers should
be cc'd to further the inclussion of commit ad7675b15fd87f1 -- Nikos do
you?

Thanks,
Mike