Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

Tejun Heo
2019-11-07T15:44:04Z

On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin wrote:
> We've encountered a rcu stall in get_mem_cgroup_from_mm():
>
> rcu: INFO: rcu_sched self-detected stall on CPU
> rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
> (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
> <...>
> RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
> <...>
> __memcg_kmem_charge+0x55/0x140
> __alloc_pages_nodemask+0x267/0x320
> pipe_write+0x1ad/0x400
> new_sync_write+0x127/0x1c0
> __kernel_write+0x4f/0xf0
> dump_emit+0x91/0xc0
> writenote+0xa0/0xc0
> elf_core_dump+0x11af/0x1430
> do_coredump+0xc65/0xee0
> ? unix_stream_sendmsg+0x37d/0x3b0
> get_signal+0x132/0x7c0
> do_signal+0x36/0x640
> ? recalc_sigpending+0x17/0x50
> exit_to_usermode_loop+0x61/0xd0
> do_syscall_64+0xd4/0x100
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The problem is caused by an exiting task which is associated with
> an offline memcg. We're iterating over and over in the
> do {} while (!css_tryget_online()) loop, but obviously the memcg won't
> become online and the exiting task won't be migrated to a live memcg.
>
> Let's fix it by switching from css_tryget_online() to css_tryget().
>
> As css_tryget_online() cannot guarantee that the memcg won't go
> offline, the check is usually useless, except some rare cases
> when for example it determines if something should be presented
> to a user.
>
> A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> css_tryget() instead of css_tryget_online() in task_get_css()").
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: stable@vger.kernel.org
> Cc: Tejun Heo <tj@kernel.org>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

--
tejun