linux-kernel - Re: [PATCH 10/19] NET: set PF_FSTRANS while holding sk

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Message-ID: <1397625226.4222.113.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 15 Apr 2014 22:13:46 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	NeilBrown <neilb@...e.de>
Cc:	linux-mm@...ck.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH 10/19] NET: set PF_FSTRANS while holding sk_lock

On Wed, 2014-04-16 at 14:03 +1000, NeilBrown wrote:
> sk_lock can be taken while reclaiming memory (in nfsd for loop-back
> NFS mounts, and presumably in nfs), and memory can be allocated while
> holding sk_lock, at least via:
> 
>  inet_listen -> inet_csk_listen_start ->reqsk_queue_alloc
> 
> So to avoid deadlocks, always set PF_FSTRANS while holding sk_lock.
> 
> This deadlock was found by lockdep.

Wow, this is adding expensive stuff in fast path, only for nfsd :(

BTW, why should the current->flags should be saved on a socket field,
and not a current->save_flags. This really looks a thread property, not
a socket one.

Why nfsd could not have PF_FSTRANS in its current->flags ?

For applications handling millions of sockets, this makes a difference.


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