netdev - Re: [PATCH] xfrm: Fix RCU vs hash_resize

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Date:   Mon, 5 Jul 2021 13:58:50 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Frederic Weisbecker <frederic@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        "Ahmed S . Darwish" <a.darwish@...utronix.de>,
        <stable@...r.kernel.org>, Varad Gautam <varad.gautam@...e.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH] xfrm: Fix RCU vs hash_resize_mutex lock inversion

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@...r.kernel.org
> > Cc: Ahmed S. Darwish <a.darwish@...utronix.de>
> > Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Varad Gautam <varad.gautam@...e.com>
> > Cc: Steffen Klassert <steffen.klassert@...unet.com>
> > Cc: Herbert Xu <herbert@...dor.apana.org.au>
> > Cc: David S. Miller <davem@...emloft.net>
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> 
> 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!