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.
Also handles the case where the deferred-QS irq-work handler may run between segments of a compound section (per Paul McKenney's counter-example): I also handled expedited GP cases in a patch. I have not yet looked at how this interacts with the softirq paths so I am keeping the RFC tag on. The last patch is a debug-only detector (CONFIG_RCU_GP_CLEANUP_STALE_CHECK, marked [TEST COMMIT], not for merge) -- applied alone on unmodified mainline without patches 2-5 it reliably fires a WARN within 5 minutes under TREE03 rcutorture, confirming the bug exists and the detector catches it; with the full fix applied, I could not reproduce the issue. The git tree with all patches can be found at: https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/tag/?h=rcu-dqs-stuck-rfc-v1-20260522 Joel Fernandes (6): rcu: introduce rcu_defer_qs_clear() helper rcu: clear defer_qs_pending when notifying GP changes rcu: clear defer_qs_pending in handler for compounded sections rcu: drop redundant defer_qs_pending clear in irqrestore handler rcu: clear defer_qs_pending at expedited IPI entry [TEST COMMIT] rcu: detect stuck defer_qs_pending at GP cleanup kernel/rcu/Kconfig.debug | 11 +++++++++ kernel/rcu/tree.c | 49 ++++++++++++++++++++++++++++++++++++++++ kernel/rcu/tree.h | 13 +++++++++++ kernel/rcu/tree_exp.h | 6 +++++ kernel/rcu/tree_plugin.h | 30 ++++++++++++------------ 5 files changed, 94 insertions(+), 15 deletions(-) -- 2.34.1
