From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Benjamin Marzinski" Subject: Re: [PATCH v2 07/10] libmultipath: handle TUR threads that can't be cancelled Date: Tue, 23 Oct 2018 16:21:27 -0500 Message-ID: <20181023212127.GF7100@octiron.msp.redhat.com> References: <20181023134348.17915-1-mwilck@suse.com> <20181023134348.17915-2-mwilck@suse.com> <20181023192849.GA7100@octiron.msp.redhat.com> <8a3659b919b21305cf7ef6bae321cc3b2b93ffd6.camel@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: <8a3659b919b21305cf7ef6bae321cc3b2b93ffd6.camel@suse.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Martin Wilck Cc: mwilck+gmail@suse.de, dm-devel@redhat.com List-Id: dm-devel.ids On Tue, Oct 23, 2018 at 10:49:49PM +0200, Martin Wilck wrote: > On Tue, 2018-10-23 at 14:28 -0500, Benjamin Marzinski wrote: > = > > On Tue, Oct 23, 2018 at 03:43:45PM +0200, Martin Wilck wrote: > > > When the tur checker code determines that a hanging TUR thread > > > couldn't be cancelled, rather than simply returning, reallocate > > > the checker context and start a new thread. This will leak some > > > memory if the hanging thread never wakes up again, but well, in > > > that highly unlikely case we're leaking threads anyway. > > = > > The thing about PATH_UNCHECKED is that we do mark the path as failed. > > We just don't tell device-mapper. If we get PATH_UNCHECKED in > > pathinfo(), we set the state to PATH_DOWN. If we get a PATH_UNCHECKED > > in > > check_path(), we immediately call pathinfo(), = > = > But we call pathinfo(pp, conf, 0) there, i.e. all DI flags unset. This > comes down to a (partial) blacklist check (which is ignored) and a call > to path_offline(). pp->state isn't touched in this code path. It's more > or less a NOOP. (BTW, the purpose of this pathinfo() call remains > obscure to me. It goes back to the ancient commit 95987395). Oops. I overlooked the flags. Well, that takes care of any issue with check_path(). As for the pathinfo call itself, I assume that, at some tine, it was possible to get to check_path() without a fully initialized path device. But that doesn't explain why it doesn't set any flags. I'm not sure if this code currently serves any purpose. = > > making it likely that we > > will get PATH_UNCHECKED there as well. The consequence of this is > > that > > if the path later changes to PATH_DOWN, which seems likely, we still > > won't tell device-mapper, since as far as multipathd is concerned, > > the > > path hasn't changed state. = > = > I don't follow you. check_path() quits early when PATH_UNCHECKED is > encountered, and doesn't alter pp->state. It will check again in the > next round, and if the path switches to DOWN then (or any other state, > for that matter) it will treat it right, AFAICS. If PATH_UNCHECKED triggers the "blank" code in pathinfo, it will set the path's state to PATH_DOWN. > > Looking at most of the code, the way we > > treat PATH_UNCHECKED really only makes sense when we use it to mean > > we > > haven't ever gotten a result from get_state() before. > > = > > If you want a return code that does just does nothing with the path, > > except wait, that's PATH_PENDING. It leaves the paths state exactly > > the > > same as before. The only issue there is that we schedule another path > > checker for a second later, which might not be the right answer to an > > out-of-memory issue. > > = > > If you've reviewed the code paths that we follow on PATH_UNCHECKED, > > and > > still feel that it is the right answer, I won't block it, because > > this > > is a pretty remote corner case. > = > PATH_UNCHECKED is tested in the following places (ignoring calls from > multipath and mpathpersist): > = > 1) pathinfo(): in the "blank" case (we discussed that before, I think > it's wrong and I removed that test in 17/21 of my previous 21-part > series). removing the blank case here fixes the issue with getting a PATH_UNCHECKED in pathinfo(), which is the root of my objection to it, so Reviewed-by: Benjamin Marzinski > 2) sync_map_state(): no attempt to sync the path with dm is made, > which is what we want here, IMHO. > 3) check_path(): see above. > = > So yes, IMO the code review shows that PATH_UNCHECKED is better then > PATH_TIMEOUT for the corner case at hand. > = > Regards > Martin > = > = > > But I don't like how PATH_UNCHECKED > > works like PATH_DOWN, but without actually keeping the state synced > > with > > the kernel, since the rest of the multipathd code is expecting the > > state > > to be synced. > > = > > > Signed-off-by: Martin Wilck > > > --- > > > libmultipath/checkers/tur.c | 24 +++++++++++++++++++++--- > > > 1 file changed, 21 insertions(+), 3 deletions(-) > > > = > > > diff --git a/libmultipath/checkers/tur.c > > > b/libmultipath/checkers/tur.c > > > index 41210892..a6c88eb2 100644 > > > --- a/libmultipath/checkers/tur.c > > > +++ b/libmultipath/checkers/tur.c > > > @@ -349,11 +349,29 @@ int libcheck_check(struct checker * c) > > > } > > > } else { > > > if (uatomic_read(&ct->holders) > 1) { > > > - /* The thread has been cancelled but hasn't > > > - * quit. exit with timeout. */ > > > + /* > > > + * The thread has been cancelled but hasn't > > > quit. > > > + * We have to prevent it from interfering with > > > the new > > > + * thread. We create a new context and leave > > > the old > > > + * one with the stale thread, hoping it will > > > clean up > > > + * eventually. > > > + */ > > > condlog(3, "%d:%d : tur thread not responding", > > > major(ct->devt), minor(ct->devt)); > > > - return PATH_TIMEOUT; > > > + > > > + /* > > > + * libcheck_init will replace c->context. > > > + * It fails only in OOM situations. In this > > > case, return > > > + * PATH_UNCHECKED to avoid prematurely failing > > > the path. > > > + */ > > > + if (libcheck_init(c) !=3D 0) > > > + return PATH_UNCHECKED; > > > + > > > + if (!uatomic_sub_return(&ct->holders, 1)) > > > + /* It did terminate, eventually */ > > > + cleanup_context(ct); > > > + > > > + ct =3D c->context; > > > } > > > /* Start new TUR checker */ > > > pthread_mutex_lock(&ct->lock); > > > -- = > > > 2.19.1 > > = > > -- > > dm-devel mailing list > > dm-devel@redhat.com > > https://www.redhat.com/mailman/listinfo/dm-devel > > = > = > -- = > Dr. Martin Wilck , Tel. +49 (0)911 74053 2107 > SUSE Linux GmbH, GF: Felix Imend=F6rffer, Jane Smithard, Graham Norton > HRB 21284 (AG N=FCrnberg) > =