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).
> 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.
> 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).
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 <mwilck@xxxxxxxx>
> > ---
> > 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) != 0)
> > + return PATH_UNCHECKED;
> > +
> > + if (!uatomic_sub_return(&ct->holders, 1))
> > + /* It did terminate, eventually */
> > + cleanup_context(ct);
> > +
> > + ct = c->context;
> > }
> > /* Start new TUR checker */
> > pthread_mutex_lock(&ct->lock);
> > --
> > 2.19.1
>
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--
Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)
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