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cgroup: Use css_tryget() instead of css_tryget_online() in task_get_css()
commit18fa84a2db
upstream. 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 Signed-off-by: Tejun Heo <tj@kernel.org> Cc: stable@vger.kernel.org # v4.2+ Fixes:ec438699a9
("cgroup, block: implement task_get_css() and use it in bio_associate_current()") Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 changed files with 8 additions and 2 deletions
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@ -484,7 +484,7 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
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*
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* Find the css for the (@task, @subsys_id) combination, increment a
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* reference on and return it. This function is guaranteed to return a
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* valid css.
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* valid css. The returned css may already have been offlined.
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*/
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static inline struct cgroup_subsys_state *
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task_get_css(struct task_struct *task, int subsys_id)
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@ -494,7 +494,13 @@ task_get_css(struct task_struct *task, int subsys_id)
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rcu_read_lock();
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while (true) {
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css = task_css(task, subsys_id);
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if (likely(css_tryget_online(css)))
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/*
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* Can't use css_tryget_online() here. A task which has
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* PF_EXITING set may stay associated with an offline css.
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* If such task calls this function, css_tryget_online()
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* will keep failing.
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*/
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if (likely(css_tryget(css)))
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break;
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cpu_relax();
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}
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