lkml.org 
[lkml]   [2020]   [Dec]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] rcu/segcblist: Add debug checks for segment lengths
On Tue, Dec 01, 2020 at 05:26:32PM -0500, Joel Fernandes wrote:
> On Thu, Nov 19, 2020 at 3:42 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Thu, Nov 19, 2020 at 12:16:15PM -0800, Paul E. McKenney wrote:
> > > On Thu, Nov 19, 2020 at 02:44:35PM -0500, Joel Fernandes wrote:
> > > > On Thu, Nov 19, 2020 at 2:22 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > > > > > > > On Wed, Nov 18, 2020 at 11:15:41AM -0500, Joel Fernandes (Google) wrote:
> > > > > > > > > > After rcu_do_batch(), add a check for whether the seglen counts went to
> > > > > > > > > > zero if the list was indeed empty.
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > > > > >
> > > > > > > > > Queued for testing and further review, thank you!
> > > > > > > >
> > > > > > > > FYI, the second of the two checks triggered in all four one-hour runs of
> > > > > > > > TREE01, all four one-hour runs of TREE04, and one of the four one-hour
> > > > > > > > runs of TREE07. This one:
> > > > > > > >
> > > > > > > > WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
> > > > > > > >
> > > > > > > > That is, there are callbacks in the list, but the sum of the segment
> > > > > > > > counts is nevertheless zero. The ->nocb_lock is held.
> > > > > > > >
> > > > > > > > Thoughts?
> > > > > > >
> > > > > > > FWIW, TREE01 reproduces it very quickly compared to the other two
> > > > > > > scenarios, on all four run, within five minutes.
> > > > > >
> > > > > > So far for TREE01, I traced it down to an rcu_barrier happening so it could
> > > > > > be related to some interaction with rcu_barrier() (Just a guess).
> > > > >
> > > > > Well, rcu_barrier() and srcu_barrier() are the only users of
> > > > > rcu_segcblist_entrain(), if that helps. Your modification to that
> > > > > function looks plausible to me, but the system's opinion always overrules
> > > > > mine. ;-)
> > > >
> > > > Right. Does anything the bypass code standout? That happens during
> > > > rcu_barrier() as well, and it messes with the lengths.
> > >
> > > In theory, rcu_barrier_func() flushes the bypass before doing the
> > > entrain, and does the rcu_segcblist_entrain() afterwards.
> > >
> > > Ah, and that is the issue. If ->cblist is empty and ->nocb_bypass
> > > is not, then ->cblist length will be nonzero, and none of the
> > > segments will be nonzero.
> > >
> > > So you need something like this for that second WARN, correct?
> > >
> > > WARN_ON_ONCE(!rcu_segcblist_empty(&rdp->cblist) &&
> > > rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
>
> Just started to look into it again. If the &rdp->cblist is empty, that
> means the bypass list could not have been used (Since per comments on
> rcu_nocb_try_bypass() , the bypass list is in use only when the cblist
> is non-empty). So the cblist was non empty, then the segment counts
> should not sum to 0. So I don't think that explains it. Anyway, I
> will try the new version of your warning in case there is something
> about bypass lists that I'm missing.

Good point. I really did see failures, though. Do they show up for
you?

Thanx, Paul

\
 
 \ /
  Last update: 2020-12-02 05:22    [W:0.099 / U:1.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site