Re: [PATCH v4 0/8] rcu: fix stuck defer_qs_pending state and add rescue timer

[Joel Fernandes <joelagnelf@nvidia.com>]

On 6/25/2026 8:42 PM, Joel Fernandes wrote:
> This series fixes a bug where rdp->defer_qs_pending can remain stuck in
> PENDING when a preempted reader's quiescent state is reported up-tree via
> a path other than the deferred-QS irq-work handler (FQS scan, hotplug
> transition, expedited GP IPI, context switch). Once stuck, the pending
> gate in rcu_read_unlock_special() silently suppresses all future arming
> attempts on that CPU. The series adds PENDING -> IDLE transitions at the
> missing sites (patches 1-7), including the case where the deferred-QS
> irq-work handler may run between segments of a compound section (per Paul
> McKenney's counter-example) and the softirq deferred-QS arming path.
> 
> Patch 8 adds a per-CPU rescue hrtimer that bounds the worst-case
> deferred-QS reporting latency: when the irq-work handler lands in a clean
> (non-reader, non-compound) context it reports the quiescent state directly
> via the new rcu_preempt_deferred_qs_try_report() helper, and the rescue timer
> reuses the same helper so that, under preempt=none, the QS report is quick
> without depending on the scheduler.  The rescue timer is cancelled from the
> normal deferred-QS report path so it does not fire once the quiescent state
> has already been reported.
> 
> This version is rebased on top of Paul's latest rcu/dev branch. The
> rcutorture reader-end deboost test patches that were folded into v3 are now
> in rcu/dev and have been dropped here. The git tree below additionally
> carries two debug-only commits on top of the series ([TEST COMMIT], not for
> merge): a detector that WARNs if defer_qs_pending is stuck at GP cleanup,
> and an rcutorture tweak that gives the async deboost mechanisms up to 500us
> before warning. Applied alone on unmodified mainline, the detector reliably
> fires within 5 minutes under TREE03 rcutorture; with the full fix applied I
> could not reproduce the issue.
> 
> The git tree with all patches can be found at:
> git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (tag: rcu-dqs-stuck-v4-20260625)

Just fyi, the 2 "[TEST COMMIT]" patches not for merge are in the above tag. I
missed including it in the LKML posting, but they are not for merge anyway.