Re: [PATCH 10/19] NET: set PF_FSTRANS while holding sk_lock

Eric Dumazet
2014-04-16T05:13:50Z

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.