Tejun Heo 93618edf75 cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated ... A chain of commits going back to v7.0 reworked rmdir to satisfy the controller invariant that a subsystem's ->css_offline() must not run while tasks are still doing kernel-side work in the cgroup. [1] d245698d72 ("cgroup: Defer task cgroup unlink until after the task is done switching out") [2] a72f73c4dd ("cgroup: Don't expose dead tasks in cgroup") [3] 1b164b876c ("cgroup: Wait for dying tasks to leave on rmdir") [4] 4c56a8ac68 ("cgroup: Fix cgroup_drain_dying() testing the wrong condition") [5] 13e786b64b ("cgroup: Increment nr_dying_subsys_* from rmdir context") [1] moved task cset unlink from do_exit() to finish_task_switch() so a task's cset link drops only after the task has fully stopped scheduling. That made tasks past exit_signals() linger on cset->tasks until their final context switch, which led to a series of problems as what userspace expected to see after rmdir diverged from what the kernel needs to wait for. [2]-[5] tried to bridge that divergence: [2] filtered the exiting tasks from cgroup.procs; [3] had rmdir(2) sleep in TASK_UNINTERRUPTIBLE for them; [4] fixed the wait's condition; [5] made nr_dying_subsys_* visible synchronously. The cgroup_drain_dying() wait in [3] turned out to be a dead end. When the rmdir caller is also the reaper of a zombie that pins a pidns teardown (e.g. host PID 1 systemd reaping orphan pids that were re-parented to it during the same teardown), rmdir blocks in TASK_UNINTERRUPTIBLE waiting for those pids to free, the pids can't free because PID 1 is the reaper and it's stuck in rmdir, and the system A-A deadlocks. No internal lock ordering breaks this; the wait itself is the bug. The css killing side that drove the original reorder, however, can be made cleanly asynchronous: ->css_offline() is already async, run from css_killed_work_fn() driven by percpu_ref_kill_and_confirm(). The fix is to make that chain start only after all tasks have left the cgroup. rmdir's user-visible side then returns as soon as cgroup.procs and friends are empty, while ->css_offline() still runs only after the cgroup is fully drained. Verified by the original reproducer (pidns teardown + zombie reaper, runs under vng) which hangs vanilla and succeeds here, and by per-commit deterministic repros for [2], [3], [4], [5] with a boot parameter that widens the post-exit_signals() window so each state is reliably reachable. Some stress tests on top of that. cgroup_apply_control_disable() has the same shape of pre-existing race: when a controller is disabled via subtree_control, kill_css() ran synchronously while tasks past exit_signals() could still be linked to the cgroup's csets, and ->css_offline() could fire before they drained. This patch preserves the existing synchronous behavior at that call site (kill_css_sync() + kill_css_finish() back-to-back) and a follow-up patch will defer kill_css_finish() there using a per-css trigger. This seems like the right approach and I don't see problems with it. The changes are somewhat invasive but not excessively so, so backporting to -stable should be okay. If something does turn out to be wrong, the fallback is to revert the entire chain ([1]-[5]) and rework in the development branch instead. v2: Pin cgrp across the deferred destroy work with explicit cgroup_get()/cgroup_put() around queue_work() and the work_fn. v1 wasn't actually broken (ordered cgroup_offline_wq + queue_work order in cgroup_task_dead() saved it) but the explicit ref removes the dependency on those non-obvious invariants. Also note the pre-existing cgroup_apply_control_disable() race in the description; a follow-up will defer kill_css_finish() there. Fixes: 1b164b876c ("cgroup: Wait for dying tasks to leave on rmdir") Cc: stable@vger.kernel.org # v7.0+ Reported-and-tested-by: Martin Pitt <martin@piware.de> Link: https://lore.kernel.org/all/afHNg2VX2jy9bW7y@piware.de/ Link: https://lore.kernel.org/all/35e0670adb4abeab13da2c321582af9f@kernel.org/ Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>		2026-05-04 08:52:26 -10:00
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acpi	Power management updates for 7.1-rc1	2026-04-13 19:47:52 -07:00
asm-generic	mm.git review status for linus..mm-nonmm-stable	2026-04-16 20:11:56 -07:00
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crypto	This update includes the following changes:	2026-04-15 15:22:26 -07:00
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dt-bindings	Support for Mobileye EyeQ6Lplus	2026-04-17 08:53:23 -07:00
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linux	cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated	2026-05-04 08:52:26 -10:00
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sound	ASoC: Updates for v7.1	2026-04-13 18:09:48 +02:00
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xen	xen/grant-table: guard gnttab_suspend/resume with CONFIG_HIBERNATE_CALLBACKS	2026-04-10 11:07:21 +02:00
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