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

[Oleg Nesterov]

On 11/13, Michal Hocko wrote:
>
> 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

we don't exclude it,

> when it is not sleeping in the freezer.

Yes, it is not sleeping in __refrigerator(), but it does

	schedule();
	freezer_count();

so it will enter __refrigerator() right after wakeup. If it won't be woken
up we do not care, we can consider it "frozen".

> I can see how other threads need to be zapped and TASK_WAKEKILL doesn't
> do that but shouldn't we fix that instead?

Not sure I understand, but unlikely we can (or want) to make __refrigerator()
killable.

Otherwise, how can we fix that?

Oleg.