Re: [PATCH] percpu: Allow to kill tasks doing pcpu_alloc() and waiting for pcpu_balance_workfn()

[Andrew Morton]

On Wed, 14 Mar 2018 15:09:09 -0700 Tejun Heo <tj@xxxxxxxxxx> wrote:

> Hello, Andrew.
> 
> On Wed, Mar 14, 2018 at 01:56:31PM -0700, Andrew Morton wrote:
> > It would benefit from a comment explaining why we're doing this (it's
> > for the oom-killer).
> 
> Will add.
> 
> > My memory is weak and our documentation is awful.  What does
> > mutex_lock_killable() actually do and how does it differ from
> > mutex_lock_interruptible()?  Userspace tasks can run pcpu_alloc() and I
> 
> IIRC, killable listens only to SIGKILL.
> 
> > wonder if there's any way in which a userspace-delivered signal can
> > disrupt another userspace task's memory allocation attempt?
> 
> Hmm... maybe.  Just honoring SIGKILL *should* be fine but the alloc
> failure paths might be broken, so there are some risks.  Given that
> the cases where userspace tasks end up allocation percpu memory is
> pretty limited and/or priviledged (like mount, bpf), I don't think the
> risks are high tho.

hm.  spose so.  Maybe.  Are there other ways?  I assume the time is
being spent in pcpu_create_chunk()?  We could drop the mutex while
running that stuff and take the appropriate did-we-race-with-someone
testing after retaking it.  Or similar.