linux-kernel - Re: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup

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Message-ID: <YLEIEa6DLjgd5mu5@lx-t490>
Date:   Fri, 28 May 2021 17:11:13 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Varad Gautam <varad.gautam@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        netdev@...r.kernel.org, stable@...r.kernel.org,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Florian Westphal <fw@...len.de>
Subject: Re: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in
 xfrm_policy_lookup_bytype

On Fri, May 28, 2021, Varad Gautam wrote:
...
>
> Thead 1 (xfrm_hash_resize)	Thread 2 (xfrm_policy_lookup_bytype)
>
> 				rcu_read_lock();
> mutex_lock(&hash_resize_mutex);
> 				read_seqcount_begin(&xfrm_policy_hash_generation);
> 				mutex_lock(&hash_resize_mutex); // block
> xfrm_bydst_resize();
> synchronize_rcu(); // block
> 		<RCU stalls in xfrm_policy_lookup_bytype>
>
...
> Fixes: a7c44247f70 ("xfrm: policy: make xfrm_policy_lookup_bytype lockless")

Minor note: the 'Fixes' commit should be 77cc278f7b20 ("xfrm: policy:
Use sequence counters with associated lock") instead.

The reason read_seqcount_begin() is emitting a mutex_lock() on
PREEMPT_RT is because of the s/seqcount_t/seqcount_mutex_t/ change.

Kind regards,

--
Ahmed S. Darwish
Linutronix GmbH