Re: [PATCH] xfrm: Fix RCU vs hash_resize_mutex lock inversion

[Steffen Klassert]

On Wed, Jun 30, 2021 at 08:57:53AM +0200, Steffen Klassert wrote:
> On Mon, Jun 28, 2021 at 03:34:28PM +0200, Frederic Weisbecker wrote:
> > xfrm_bydst_resize() calls synchronize_rcu() while holding
> > hash_resize_mutex. But then on PREEMPT_RT configurations,
> > xfrm_policy_lookup_bytype() may acquire that mutex while running in an
> > RCU read side critical section. This results in a deadlock.
> > 
> > In fact the scope of hash_resize_mutex is way beyond the purpose of
> > xfrm_policy_lookup_bytype() to just fetch a coherent and stable policy
> > for a given destination/direction, along with other details.
> > 
> > The lower level net->xfrm.xfrm_policy_lock, which among other things
> > protects per destination/direction references to policy entries, is
> > enough to serialize and benefit from priority inheritance against the
> > write side. As a bonus, it makes it officially a per network namespace
> > synchronization business where a policy table resize on namespace A
> > shouldn't block a policy lookup on namespace B.
> > 
> > Fixes: 77cc278f7b20 (xfrm: policy: Use sequence counters with associated lock)
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Varad Gautam <varad.gautam@xxxxxxxx>
> > Cc: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> 
> Your patch has a conflicht with ("commit d7b0408934c7 xfrm: policy: Read
> seqcount outside of rcu-read side in xfrm_policy_lookup_bytype")
> from Varad. Can you please rebase onto the ipsec tree?

This patch is now applied to the ipsec tree (on top of the
revert of commit d7b0408934c7).

Thanks everyone!