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

[Michal Hocko]

On Tue 13-11-18 17:18:58, Oleg Nesterov wrote:
> 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".

Right, but this is just silencing the freezing code to exclude this
task, right?

> > 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.

Why would that be a problem. If the kill is fatal then why to keep the
killed task in the fridge?

> Otherwise, how can we fix that?

We can mark all threads PF_NOFREEZE and wake them up. This would require
some more changes of course but wouldn't that be a more appropriate
solution? Do we want to block exec for ever just because some threads
are in the fridge?

-- 
Michal Hocko
SUSE Labs