On Mon, Nov 20 2017 at 8:35pm -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > On Mon, Nov 20 2017 at 7:34pm -0500, > NeilBrown <neilb@xxxxxxxx> wrote: > > > On Mon, Nov 20 2017, Mike Snitzer wrote: > > > > > > > > But I've now queued this patch for once Linus gets back (reverts DM > > > changes from commit 47e0fb461f): > > > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c9fdc42ba23eabd1ba7aef199fb9bb4b4fe5c545 > > > > This patch does two things. > > 1/ It removes the BIOSET_NEED_RESCUER flag from biosets created by dm. > > This a functional changed over the code from before my patches. > > Previously, all biosets were given a rescuer thread. > > After my patch set, biosets only got a rescuer thread if > > BIOSET_NEED_RESCUER was passed, and it was passed for all biosets. > > I then removed it from places were I was certain it wasn't needed. > > I didn't remove it from dm because I wasn't certain. Your > > patch does remove the flags, which I think is incorrect - see below. Yeap, definitely was incorrect. I've dropped the patch. > > 2/ It changes flush_current_bio_list() so that bios allocated from a > > bioset that does not have a rescue_workqueue are now added to > > the ->rescue_list for their bio_set, and ->rescue_work is queued > > on the NULL ->rescue_workqueue, resulting in a NULL dereference. > > I suspect you don't want this. Yes, I see that now. > > The patch description claims that the patch fixes something, but it > > isn't clear to me what it is meant to be fixing. > > > > It makes reference to dbba42d8 which is described as removing an unused > > bioset process, though what it actually does is remove an used bioset > > (and obvious the process disappears with it). My patch doesn't change > > that behavior. > > Well I looked at this because Zdenek reported that with more recent > kernels he is seeing the "bioset" per DM device again (whereas it was > thought to be removed with mikulas' commit dbba42d8 -- but that commit > removed "bioset" only in terms of q->bio_split. I think Zdenek triggered a false-positive that DM had magically sprouted a new "bioset" rescue_workqueue. Reality is I cannot see how each bio-based DM device can avoid having one. And the commit d67a5f4b59 ("dm: flush queued bios when process blocks to avoid deadlock") I referenced earlier very much makes DM depend on it even more. So apologies for being so off-base (by looking to prematurely revert DM's use of BIOSET_NEED_RESCUER, etc). > > Please see > > https://www.redhat.com/archives/dm-devel/2017-August/msg00310.html I'll very likely pick these up for 4.16 shortly. But hope to work through complete removal of DM's use of BIOSET_NEED_RESCUER for 4.16 as well. > > and > > https://www.redhat.com/archives/dm-devel/2017-August/msg00315.html This one [1] needs a lot of review and testing. Particularly against this test case that Mikulas created to reproduce the snapshot deadlock (same deadlock that motivated commit dbba42d8): https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html > > for which the thread continues: > > https://www.redhat.com/archives/dm-devel/2017-September/msg00001.html Wish I could clone myself (or Kent, the world needs 2 Kents!) and pursue this: https://www.redhat.com/archives/dm-devel/2014-May/msg00100.html Short of that, how would you like to proceed? > > That would then just leave bcache.... I find it a bit of a challenge to > > reason about the code in bcache, but if we can remove > > BIOSET_NEED_RESCUER from dm, that will be an extra incentive for me to learn :-) > > I'm all for properly removing BIOSET_NEED_RESCUER from DM. Should we work to make [1] (above) sure it fixes Mikulas' test case? I'll set in on reviewing and playing with [1] now. Thanks, Mike
