Re: [PATCH v2] exec: make de_thread() freezable

[Michal Hocko <mhocko@kernel.org>]

On Mon 12-11-18 12:54:45, Chanho Min wrote:
> Suspend fails due to the exec family of functions blocking the freezer.
> The casue is that de_thread() sleeps in TASK_UNINTERRUPTIBLE waiting for
> all sub-threads to die, and we have the deadlock if one of them is frozen.
> This also can occur with the schedule() waiting for the group thread leader
> to exit if it is frozen.
> 
> In our machine, it causes freeze timeout as bellows.
> 
> Freezing of tasks failed after 20.010 seconds (1 tasks refusing to freeze, wq_busy=0):
> setcpushares-ls D ffffffc00008ed70     0  5817   1483 0x0040000d
>  Call trace:
> [<ffffffc00008ed70>] __switch_to+0x88/0xa0
> [<ffffffc000d1c30c>] __schedule+0x1bc/0x720
> [<ffffffc000d1ca90>] schedule+0x40/0xa8
> [<ffffffc0001cd784>] flush_old_exec+0xdc/0x640
> [<ffffffc000220360>] load_elf_binary+0x2a8/0x1090
> [<ffffffc0001ccff4>] search_binary_handler+0x9c/0x240
> [<ffffffc00021c584>] load_script+0x20c/0x228
> [<ffffffc0001ccff4>] search_binary_handler+0x9c/0x240
> [<ffffffc0001ce8e0>] do_execveat_common.isra.14+0x4f8/0x6e8
> [<ffffffc0001cedd0>] compat_SyS_execve+0x38/0x48
> [<ffffffc00008de30>] el0_svc_naked+0x24/0x28
> 
> To fix this, make de_thread() freezable. It looks safe and works fine.

It's been some time since I have looked into this code so bear with me.
One thing is not really clear to me. Why does it help to exclude this
particular task from the freezer when it is not sleeping in the freezer.
I can see how other threads need to be zapped and TASK_WAKEKILL doesn't
do that but shouldn't we fix that instead?

Or maybe I am missing something important here.
-- 
Michal Hocko
SUSE Labs