On Fri, 15 Jan 2016, Hannes Frederic Sowa wrote:

On 14.01.2016 23:20, Eric Dumazet wrote:

quoted

On Thu, 2016-01-14 at 23:02 +0100, Hannes Frederic Sowa wrote:

quoted

We are just adding a second recursion limit solely to openvswitch which
has the same problem:

https://patchwork.ozlabs.org/patch/566769/

This time also we depend on rcu_read_lock marking the section being
nonpreemptible. Nice would be a more generic solution here which doesn't
need to always add something to *current.

Note that rcu_read_lock() does not imply that preemption is disabled.

Exactly, it is conditional on CONFIG_PREEMPT_CPU/CONFIG_PREMPT_COUNT but
haven't thought about exactly that in this moment.

Wrong. CONFIG_PREEMPT_RCU makes RCU preemptible.

If that is not set then it fiddles with preempt_count when
CONFIG_PREEMPT_COUNT=y. If CONFIG_PREEMPT_COUNT=n then you have a non
preemptible system anyway.

So you cannot assume that rcu_read_lock() disables preemption.

Thanks,

	tglx