linux-kernel - Re: [PATCH] xfrm: Fix RCU vs hash_resize

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Date:   Wed, 30 Jun 2021 14:21:05 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Varad Gautam <varad.gautam@...e.com>
CC:     Frederic Weisbecker <frederic@...nel.org>,
        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>, 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 02:11:24PM +0200, Varad Gautam wrote:
> On 6/30/21 8:57 AM, 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?
> > 
> > Btw. Varad, your above mentioned patch tried to fix the same issue.
> > Do we still need it, or is it obsolete with the fix from Frederic?
> > 
> 
> The patch "xfrm: policy: Read seqcount outside of rcu-read side in
> xfrm_policy_lookup_bytype" shouldn't be needed after Frederic's fix since
> the offending mutex is now gone. It can be dropped.

Ok, so I'll revert your patch and apply Frederic's patch on
top of that revert.

Thanks a lot everyone!