Commit 18fa84a2 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: Use css_tryget() instead of css_tryget_online() in task_get_css()



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: default avatarTejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v4.2+
Fixes: ec438699 ("cgroup, block: implement task_get_css() and use it in bio_associate_current()")
parent 9fb67d64
......@@ -487,7 +487,7 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
*
* Find the css for the (@task, @subsys_id) combination, increment a
* reference on and return it. This function is guaranteed to return a
* valid css.
* valid css. The returned css may already have been offlined.
*/
static inline struct cgroup_subsys_state *
task_get_css(struct task_struct *task, int subsys_id)
......@@ -497,7 +497,13 @@ task_get_css(struct task_struct *task, int subsys_id)
rcu_read_lock();
while (true) {
css = task_css(task, subsys_id);
if (likely(css_tryget_online(css)))
/*
* Can't use css_tryget_online() here. A task which has
* PF_EXITING set may stay associated with an offline css.
* If such task calls this function, css_tryget_online()
* will keep failing.
*/
if (likely(css_tryget(css)))
break;
cpu_relax();
}
......
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