Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

Michal Hocko
2019-10-10T07:40:53Z

On Thu 10-10-19 14:12:01, Sergey Senozhatsky wrote:
> On (10/09/19 16:26), Michal Hocko wrote:
> > On Wed 09-10-19 15:56:32, Peter Oberparleiter wrote:
> > [...]
> > > A generic solution would be preferable from my point of view though,
> > > because otherwise each console driver owner would need to ensure that any
> > > lock taken in their console.write implementation is never held while
> > > memory is allocated/released.
> >
> > Considering that console.write is called from essentially arbitrary code
> > path IIUC then all the locks used in this path should be pretty much
> > tail locks or console internal ones without external dependencies.
>
> That's a good expectation, but I guess it's not always the case.
>
> One example might be NET console - net subsystem locks, net device
> drivers locks, maybe even some MM locks (skb allocations?).

I am not familiar with the netconsole code TBH. If there is absolutely
no way around that then we might have to bite a bullet and consider some
of MM locks a land of no printk. I have already said that in this
thread. I am mostly pushing back on "let's just go the simplest way"
approach.

> But even more "commonly used" consoles sometimes break that
> expectation. E.g. 8250
>
> serial8250_console_write()
> serial8250_modem_status()
> wake_up_interruptible()

By that expectation you mean they are using external locks or that they
really _need_ to allocate. Because if you are pointing to
wake_up_interruptible and therefore the rq then this is a well known
thing and I was under impression even documented but I can only see
LOGLEVEL_SCHED that is arguably a very obscure way to document the fact.

--
Michal Hocko
SUSE Labs