Re: rcu stalls during fstests runs for xfs

[Joel Fernandes <joelagnelf@nvidia.com>]

On Fri, Feb 13, 2026 at 01:26:32AM +0000, Shinichiro Kawasaki wrote:
> Cc+: Thomas,
> 
> On Jan 30, 2026 / 20:16, Shin'ichiro Kawasaki wrote:
> > On Jan 29, 2026 / 15:19, Paul E. McKenney wrote:
> > [...]
> > > And Thomas Gleixner posted an alleged fix to the CID issue here:
> > > 
> > > https://lore.kernel.org/lkml/20260129210219.452851594@kernel.org/
> > > 
> > > Please let him know whether or not it helps.
> > 
> > Good to see this fix candidate series, thanks :) I have set up the patches and
> > started my regular test runs. So far, the hangs have been observed once or twice
> > a week. To confirm the effect of the fix series, I think two weeks runs will be
> > required. Once I get the result, will share it on this thread and with Thomas.
> 
> Two weeks have passed, and I did not observed the hang! Then I'm confident that
> the v1 fix series by Thomas avoided the rcu stall issue in my xfs-zoned test
> systems. The series is already in v6.19 kernel tag as v2. Great.
> 
> Thomas, just FYI.
> 
> I faced mysterious kernel hangs during my regular fstests runs [1]. RCU experts
> suggested the hangs might be caused by the recent MMCID changes. I tried your v1
> fix patch series "sched/mmcid: Cure mode transition woes", and confirmed it
> avoids the hangs. Thank you for the fix. The v2 series is already in v6.19
> kernel tag, so this report might not be so valuable, but just in case. (And
> thank you again for the additional quick fix for my blktests failure caused by
> one of the patches in the series).
> 
> [1] https://lore.kernel.org/rcu/aXdO52wh2rqTUi1E@shinmob/

Good to see that this got resolved. I am guessing there's nothing from an RCU
point of view that could be done differently to diagnose this earlier, since
I pretty quickly spotted it was MMCID related when I saw the existing report.
Let me/us RCU folk know if there's anything else to do here though..

thanks,

--
Joel Fernandes