On Tue, Nov 21 2017 at 2:44pm -0500, NeilBrown <neilb@xxxxxxxx> wrote: > On Tue, Nov 21 2017, Mike Snitzer wrote: > > > 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 > > Thanks for that link. I'll try to make time to experiment with the test > code and confirm my proposed approach doesn't break it. > > > > >> > 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 > > In that email Kent mentions "punt off to a per request_queue workqueue". > > That "per request_queue workqueue" is what I'm trying to get rid of. I > don't think this is a good direction. > > > > > Short of that, how would you like to proceed? > > I'd like to confirm that my approach > 1/ doesn't re-introduce a deadlock > 2/ doesn't hurt performance > and then merge it. > > Though to be honest, I don't recall exactly what "my approach" is. > Your next email picks out two important patches which probably cover > it. If/when I get to do the testing I'll let you know how it goes. I _think_ I've done the heavy lifting of what you likely had in mind ( please see: https://lkml.org/lkml/2017/11/21/567 ) Now what is left is another once-over from you to verify you're happy with the code and patch headers, etc. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
