Commits
Tejun Heo committed 18fa84a2db0
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: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v4.2+
Fixes: ec438699a9ae ("cgroup, block: implement task_get_css() and use it in bio_associate_current()")