Re: [PATCH v3.10-rc7] net: fix kernel deadlock with interface rename and netdev name retrieval.

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-06-26 at 17:23 +0200, Nicolas Schichan wrote:
> When the kernel (compiled with CONFIG_PREEMPT=n) is performing the
> rename of a network interface, it can end up waiting for a workqueue
> to complete. If userland is able to invoke a SIOCGIFNAME ioctl or a
> SO_BINDTODEVICE getsockopt in between, the kernel will deadlock due to
> the fact that read_secklock_begin() will spin forever waiting for the
> writer process (the one doing the interface rename) to update the
> devnet_rename_seq sequence.
> 
> This patch fixes the problem by adding a helper (netdev_get_name())
> and using it in the code handling the SIOCGIFNAME ioctl and
> SO_BINDTODEVICE setsockopt.
> 
> The netdev_get_name() helper uses raw_seqcount_begin() to avoid
> spinning forever, waiting for devnet_rename_seq->sequence to become
> even. cond_resched() is used in the contended case, before retrying
> the access to give the writer process a chance to finish.
> 
> The use of raw_seqcount_begin() will incur some unneeded work in the
> reader process in the contended case, but this is better than
> deadlocking the system.
> 
> Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
> ---

This goes back to commit c91f6df2db49
("sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface
name") in linux-3.8

Acked-by: Eric Dumazet <edumazet@google.com>

Thanks Nicolas