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

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Date:   Thu, 10 Oct 2019 10:37:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Peter Oberparleiter <oberpar@...ux.ibm.com>, Qian Cai <cai@....pw>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Petr Mladek <pmladek@...e.com>, akpm@...ux-foundation.org,
        rostedt@...dmis.org, peterz@...radead.org, linux-mm@...ck.org,
        john.ogness@...utronix.de, david@...hat.com,
        linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

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