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

Michal Hocko
2019-10-10T08:37:14Z

On Thu 10-10-19 17:16:29, Sergey Senozhatsky wrote:
> On (10/10/19 09:40), Michal Hocko wrote:
> [..]
> > > > 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.
>
> So this is what netconsole does (before we pass on udp to net device
> driver code, which *may be* can do more allocations, I don't know):
>
> write_msg()
> netpoll_send_udp()
> find_skb()
> alloc_skb(len, GFP_ATOMIC)
> kmem_cache_alloc_node()
>
> You are the right person to ask this question to - how many MM
> locks are involved when we do GFP_ATOMIC kmem_cache allocation?
> Is there anything to be concerned about?

At least zone->lock might involved. Maybe even more.
--
Michal Hocko
SUSE Labs