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This patch series addresses a few pre-existing memory safety and list traversal concurrency issues in scheduler debugfs handlers, and introduces per-CPU debugfs files under /sys/kernel/debug/sched/cpu/cpu/debug. Patch 1 introduces a prerequisite patch that annotates struct rq's rd (root_domain) pointer with __rcu in kernel/sched/sched.h and introduces the rcu_dereference_root_domain() helper macro (i.e., mirroring rcu_dereference_sched_domain()), updating readers across the scheduler subsystem to ensure Sparse compliance, correct memory ordering, and clean Lockdep-RCU validation under both sched_domains_mutex and RCU-sched contexts. Patch 2 fixes a use-after-free in print_dl_rq() where cpu_rq(cpu)->rd is dereferenced locklessly to display deadline bandwidth statistics. During CPU hot-unplug or cgroup cpuset repartitioning events, partition_sched_domains() calls rq_attach_root() to detach the CPU from its root_domain and schedules free_rootdomain() via call_rcu(). Without an RCU read lock, an RCU grace period can resolve concurrently while debugfs reads the file, allowing free_rootdomain() to execute kfree() and causing a use-after-free when reading dl_bw->bw. This patch adds rcu_assign_pointer() on the writer side in rq_attach_root() and uses guard(rcu)() with rcu_dereference() in print_dl_rq(). Patch 3 fixes a potential use-after-free in print_cpu() where rq->curr is dereferenced locklessly to output the running task's PID. If the task exits concurrently and its reference count drops to zero, put_task_struct() schedules __put_task_struct_rcu_cb() via call_rcu(). Without holding an RCU read lock, an RCU grace period can elapse concurrently and free the task structure via free_task(), leading to a use-after-free race condition. This patch protects rq->curr access using rcu_dereference() inside an RCU read-side critical section. Patch 4 fixes both a time-of-check to time-of-use race condition and a potential use-after-free in sched_show_numa(), where p->mm is checked locklessly and then passed to P(mm->numa_scan_seq). If the task exits concurrently via exit_mm(p), current->mm is set to NULL under task_lock(p) before mmput() is called to free the struct mm_struct. Wrapping the p->mm check and dereference in task_lock(p) eliminates both hazards. Patch 5 fixes an RCU traversal violation in print_cfs_stats() where rq->leaf_cfs_rq_list is traversed locklessly using for_each_leaf_cfs_rq_safe(), which expands to list_for_each_entry_safe(). Although leaf_cfs_rq_list is modified using list_add_rcu(), list_for_each_entry_safe() lacks READ_ONCE() and pre-fetches the next pointer without memory barriers. Furthermore, because cfs_rq nodes are re-linked on enqueue/dequeue without waiting for RCU grace periods, concurrent list churn can cause backward jumps or infinite loops. This patch introduces for_each_leaf_cfs_rq_rcu() using guard(rcu)(), bounds traversal with a per-CPU circuit-breaker ceiling, and emits an explicit truncation notice if the ceiling is reached. Patch 6 introduces per-CPU debugfs entries under /sys/kernel/debug/sched/cpu/cpu/debug, allowing targeted inspection of an individual CPU's runqueue on demand. If the target CPU is currently offline, reading its file returns -ENODEV. Changes since v9: - Updated rq_attach_root() to use rcu_assign_pointer() when assigning rq->rd - Linked to v9: https://lore.kernel.org/lkml/20261029132511.712391-1-atomlin@atomlin.com/ Changes since v8: - Added guard(rcu)() to dl_task_needs_bw_move() to prevent false-positive warnings under CONFIG_PROVE_RCU when invoked from cpuset_can_attach() - Converted remaining direct rq->rd accessors across in add_nr_running(), __sched_setscheduler(), and dl_task_check_affinity() to use rcu_dereference_root_domain() - Linked to v8: https://lore.kernel.org/lkml/20260827194014.977758-1-atomlin@atomlin.com/ Changes since v7: - Removed incorrect rcu_dereference_protected() usages with lockdep_is_held(&rq->__lock) in favour of a newly introduced rcu_dereference_root_domain() helper, properly validating access under either sched_domains_mutex or RCU-sched read-side critical sections (Peter Zijlstra) - Restored the comment in check_update_overutilized_status() in kernel/sched/fair.c (Vincent Guittot) - Linked to v7: https://lore.kernel.org/lkml/20260826224238.936456-1-atomlin@atomlin.com/ Changes since v6: - Updated rq->rd accessors across kernel/sched/ to use rcu_dereference_sched() for lockless readers and rcu_dereference_protected() for lock-held paths, eliminating false Lockdep-RCU warnings under CONFIG_PROVE_RCU and all Sparse noderef warnings - Removed the __printf(3, 4) attribute from the !CONFIG_CGROUP_SCHED fallback stub of SEQ_printf_task_group_path() in kernel/sched/sched.h to prevent -Wformat build errors when CONFIG_CGROUP_SCHED is disabled - Expanded the commit changelog to clarify the per-CPU scope of SCHED_DEBUG_MAX_ITER and the RCU memory safety guarantees for recycled cfs_rq nodes - Linked to v6: https://lore.kernel.org/lkml/20260825184637.888364-1-atomlin@atomlin.com/ Changes since v5: - Rebased against tip/sched/core (sched-core-2026-08-17) - Linked to v5: https://lore.kernel.org/lkml/20260825141413.868997-1-atomlin@atomlin.com/ Changes since v4: - Added a new prerequisite patch to annotate struct rq's rd field with __rcu and updated lockless readers to use rcu_dereference()/rcu_dereference_sched() - Updated print_dl_rq() to use guard(rcu)() and rcu_dereference() on rq->rd (Daniel Vacek and K Prateek Nayak) - Replaced READ_ONCE(p->mm) with task_lock(p)/task_unlock(p) in sched_show_numa() to prevent use-after-free against concurrent exit_mm() and mmput() - Updated print_cfs_stats() to use guard(rcu)() - Increased SCHED_DEBUG_MAX_ITER from 1024 to 4096 and added an explicit truncation notice - Moved SEQ_printf() and SEQ_printf_task_group_path() to kernel/sched/sched.h, replaced strcpy() with strscpy(), and used IS_ENABLED(CONFIG_FAIR_GROUP_SCHED) with a typed static inline fallback stub - Corrected the "Fixes:" commit tag in Patch 5 to 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path") - Linked to v4: https://lore.kernel.org/lkml/20260810015812.428999-1-atomlin@atomlin.com/ Changes since v3: - Updated Patch 1 to use rcu_dereference(rq->curr) instead of READ_ONCE() to preserve __rcu - Added missing writer-side RCU publication barrier (rcu_assign_pointer()) in rq_attach_root() for Patch 2 - Added Patch 3 to fix a TOCTOU condition in sched_show_numa() using READ_ONCE(p->mm) - Added a safety iteration ceiling in print_cfs_stats() for Patch 4 to prevent unbounded list iteration and RCU stalls under heavy leaf_cfs_rq_list churn - Linked to v3: https://lore.kernel.org/lkml/20260808235522.380038-1-atomlin@atomlin.com/ Changes since v2: - Protected lockless rq->curr dereferencing in print_cpu() with rcu_read_lock() and READ_ONCE() - Protected lockless rq->rd dereferencing in print_dl_rq() against CPU hot-unplug and cgroup cpuset repartitioning races - Introduced for_each_leaf_cfs_rq_rcu() using list_for_each_entry_rcu() for lockless leaf_cfs_rq_list iteration - Linked to v2: https://lore.kernel.org/lkml/20260728205238.18447-1-atomlin@atomlin.com/ Changes since v1: - Reframed commit message motivation around targeted interactive debugging on large SMP topologies (Peter Zijlstra and Zhan Xusheng) - Gated sched_debug_cpu_show() with a cpu_online(cpu) check returning -ENODEV when target CPU is offline (Zhan Xusheng) - Linked to v1: https://lore.kernel.org/lkml/20260728020309.6169-1-atomlin@atomlin.com/ Aaron Tomlin (6): sched: Annotate rq->rd with __rcu and update lockless readers sched/debug: Protect lockless rq->rd access in print_dl_rq() sched/debug: Protect lockless rq->curr access in print_cpu() sched/debug: Protect p->mm access in sched_show_numa() sched/fair: Use list_for_each_entry_rcu() in print_cfs_stats() sched/debug: Introduce per-CPU debugfs files kernel/sched/core.c | 24 +++++++---- kernel/sched/deadline.c | 77 ++++++++++++++++++---------------- kernel/sched/debug.c | 92 ++++++++++++++++++++++++----------------- kernel/sched/fair.c | 60 ++++++++++++++++++++------- kernel/sched/rt.c | 64 ++++++++++++++++------------ kernel/sched/sched.h | 58 +++++++++++++++++++++++++- kernel/sched/syscalls.c | 8 ++-- kernel/sched/topology.c | 13 +++--- 8 files changed, 261 insertions(+), 135 deletions(-) -- 2.55.0