Messages in this thread Patch in this message | | | From | Wander Lairson Costa <> | Subject | [PATCH v9] kernel/fork: beware of __put_task_struct calling context | Date | Tue, 16 May 2023 16:14:41 -0300 |
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Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping locks. Therefore, it can't be called from an non-preemptible context.
One practical example is splat inside inactive_task_timer(), which is called in a interrupt context:
CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W --------- Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012 Call Trace: dump_stack_lvl+0x57/0x7d mark_lock_irq.cold+0x33/0xba ? stack_trace_save+0x4b/0x70 ? save_trace+0x55/0x150 mark_lock+0x1e7/0x400 mark_usage+0x11d/0x140 __lock_acquire+0x30d/0x930 lock_acquire.part.0+0x9c/0x210 ? refill_obj_stock+0x3d/0x3a0 ? rcu_read_lock_sched_held+0x3f/0x70 ? trace_lock_acquire+0x38/0x140 ? lock_acquire+0x30/0x80 ? refill_obj_stock+0x3d/0x3a0 rt_spin_lock+0x27/0xe0 ? refill_obj_stock+0x3d/0x3a0 refill_obj_stock+0x3d/0x3a0 ? inactive_task_timer+0x1ad/0x340 kmem_cache_free+0x357/0x560 inactive_task_timer+0x1ad/0x340 ? switched_from_dl+0x2d0/0x2d0 __run_hrtimer+0x8a/0x1a0 __hrtimer_run_queues+0x91/0x130 hrtimer_interrupt+0x10f/0x220 __sysvec_apic_timer_interrupt+0x7b/0xd0 sysvec_apic_timer_interrupt+0x4f/0xd0 ? asm_sysvec_apic_timer_interrupt+0xa/0x20 asm_sysvec_apic_timer_interrupt+0x12/0x20 RIP: 0033:0x7fff196bf6f5
Instead of calling __put_task_struct() directly, we defer it using call_rcu(). A more natural approach would use a workqueue, but since in PREEMPT_RT, we can't allocate dynamic memory from atomic context, the code would become more complex because we would need to put the work_struct instance in the task_struct and initialize it when we allocate a new task_struct.
Changelog =========
v1: * Initial implementation fixing the splat.
v2: * Isolate the logic in its own function. * Fix two more cases caught in review.
v3: * Change __put_task_struct() to handle the issue internally.
v4: * Explain why call_rcu() is safe to call from interrupt context.
v5: * Explain why __put_task_struct() doesn't conflict with put_task_sruct_rcu_user.
v6: * As per Sebastian's review, revert back the implementation of v2 with a distinct function. * Add a check in put_task_struct() to warning when called from a non-sleepable context. * Address more call sites.
v7: * Fix typos. * Add an explanation why the new function doesn't conflict with delayed_free_task().
v8: * Bring back v5. * Fix coding style.
v9: * Reorganize to not need ___put_task_struct() by Oleg's suggestion.
Signed-off-by: Wander Lairson Costa <wander@redhat.com> Reported-by: Hu Chunyu <chuhu@redhat.com> Suggested-by: Oleg Nesterov <oleg@redhat.com> Suggested-by: Valentin Schneider <vschneid@redhat.com> Suggested-by: Peter Zijlstra <peterz@infradead.org> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Paul McKenney <paulmck@kernel.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Luis Goncalves <lgoncalv@redhat.com> --- include/linux/sched/task.h | 28 +++++++++++++++++++++++++++- kernel/fork.c | 8 ++++++++ 2 files changed, 35 insertions(+), 1 deletion(-)
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index d6c48163c6de..9bcb9535d4e1 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -112,10 +112,36 @@ static inline struct task_struct *get_task_struct(struct task_struct *t) } extern void __put_task_struct(struct task_struct *t); +extern void __put_task_struct_rcu_cb(struct rcu_head *rhp); static inline void put_task_struct(struct task_struct *t) { - if (refcount_dec_and_test(&t->usage)) + if (!refcount_dec_and_test(&t->usage)) + return; + + /* + * under PREEMPT_RT, we can't call put_task_struct + * in atomic context because it will indirectly + * acquire sleeping locks. + * + * call_rcu() will schedule delayed_put_task_struct_rcu() + * to be called in process context. + * + * __put_task_struct() is called when + * refcount_dec_and_test(&t->usage) succeeds. + * + * This means that it can't "conflict" with + * put_task_struct_rcu_user() which abuses ->rcu the same + * way; rcu_users has a reference so task->usage can't be + * zero after rcu_users 1 -> 0 transition. + * + * delayed_free_task() also uses ->rcu, but it is only called + * when it fails to fork a process. Therefore, there is no + * way it can conflict with put_task_struct(). + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible()) + call_rcu(&t->rcu, __put_task_struct_rcu_cb); + else __put_task_struct(t); } diff --git a/kernel/fork.c b/kernel/fork.c index 08969f5aa38d..fd3bb4a554c4 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -846,6 +846,14 @@ void __put_task_struct(struct task_struct *tsk) } EXPORT_SYMBOL_GPL(__put_task_struct); +void __put_task_struct_rcu_cb(struct rcu_head *rhp) +{ + struct task_struct *task = container_of(rhp, struct task_struct, rcu); + + __put_task_struct(task); +} +EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb); + void __init __weak arch_task_cache_init(void) { } /* -- 2.40.1
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