A PF_EXITING task can stay associated with an offline css. If such task calls task_get_css(), it can get stuck indefinitely. This can be triggered by BSD process accounting which writes to a file with PF_EXITING set when racing against memcg disable as in the backtrace at the end. After this change, task_get_css() may return a css which was already offline when the function was called. None of the existing users are affected by this change. INFO: rcu_sched self-detected stall on CPU INFO: rcu_sched detected stalls on CPUs/tasks: ... NMI backtrace for cpu 0 ... Call Trace: <IRQ> dump_stack+0x46/0x68 nmi_cpu_backtrace.cold.2+0x13/0x57 nmi_trigger_cpumask_backtrace+0xba/0xca rcu_dump_cpu_stacks+0x9e/0xce rcu_check_callbacks.cold.74+0x2af/0x433 update_process_times+0x28/0x60 tick_sched_timer+0x34/0x70 __hrtimer_run_queues+0xee/0x250 hrtimer_interrupt+0xf4/0x210 smp_apic_timer_interrupt+0x56/0x110 apic_timer_interrupt+0xf/0x20 </IRQ> RIP: 0010:balance_dirty_pages_ratelimited+0x28f/0x3d0 ... btrfs_file_write_iter+0x31b/0x563 __vfs_write+0xfa/0x140 __kernel_write+0x4f/0x100 do_acct_process+0x495/0x580 acct_process+0xb9/0xdb do_exit+0x748/0xa00 do_group_exit+0x3a/0xa0 get_signal+0x254/0x560 do_signal+0x23/0x5c0 exit_to_usermode_loop+0x5d/0xa0 prepare_exit_to_usermode+0x53/0x80 retint_user+0x8/0x8
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| @@ -487,7 +487,7 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task, · 486 unmodified lines | ||
| 487 | 487 | * |
| 488 | 488 | * Find the css for the (@task, @subsys_id) combination, increment a |
| 489 | 489 | * reference on and return it. This function is guaranteed to return a |
| 490 | − * valid css. | |
| 490 | + * valid css. The returned css may already have been offlined. | |
| 491 | 491 | */ |
| 492 | 492 | static inline struct cgroup_subsys_state * |
| 493 | 493 | task_get_css(struct task_struct *task, int subsys_id) |
| @@ -497,7 +497,13 @@ task_get_css(struct task_struct *task, int subsys_id) · 3 unmodified lines | ||
| 497 | 497 | rcu_read_lock(); |
| 498 | 498 | while (true) { |
| 499 | 499 | css = task_css(task, subsys_id); |
| 500 | − if (likely(css_tryget_online(css))) | |
| 500 | + /* | |
| 501 | + * Can't use css_tryget_online() here. A task which has | |
| 502 | + * PF_EXITING set may stay associated with an offline css. | |
| 503 | + * If such task calls this function, css_tryget_online() | |
| 504 | + * will keep failing. | |
| 505 | + */ | |
| 506 | + if (likely(css_tryget(css))) | |
| 501 | 507 | break; |
| 502 | 508 | cpu_relax(); |
| 503 | 509 | } |