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From: Varad Gautam <varad.gautam@suse.com>
To: linux-kernel@vger.kernel.org
Cc: varad.gautam@suse.com,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	netdev@vger.kernel.org, stable@vger.kernel.org,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Florian Westphal <fw@strlen.de>
Subject: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype
Date: Fri, 28 May 2021 14:03:57 +0200	[thread overview]
Message-ID: <20210528120357.29542-1-varad.gautam@suse.com> (raw)

xfrm_policy_lookup_bytype loops on seqcount mutex xfrm_policy_hash_generation
within an RCU read side critical section. Although ill advised, this is fine if
the loop is bounded.

xfrm_policy_hash_generation wraps mutex hash_resize_mutex, which is used to
serialize writers (xfrm_hash_resize, xfrm_hash_rebuild). This is fine too.

On PREEMPT_RT=y, the read_seqcount_begin call within xfrm_policy_lookup_bytype
emits a mutex lock/unlock for hash_resize_mutex. Mutex locking is fine, since
RCU read side critical sections are allowed to sleep with PREEMPT_RT.

xfrm_hash_resize can, however, block on synchronize_rcu while holding
hash_resize_mutex.

This leads to the following situation on PREEMPT_RT, where the writer is
blocked on RCU grace period expiry, while the reader is blocked on a lock held
by the writer:

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>

Move the read_seqcount_begin call outside of the RCU read side critical section,
and do an rcu_read_unlock/retry if we got stale data within the critical section.

On non-PREEMPT_RT, this shortens the time spent within RCU read side critical
section in case the seqcount needs a retry, and avoids unbounded looping.

Fixes: a7c44247f70 ("xfrm: policy: make xfrm_policy_lookup_bytype lockless")
Signed-off-by: Varad Gautam <varad.gautam@suse.com>
Cc: linux-rt-users <linux-rt-users@vger.kernel.org>
Cc: netdev@vger.kernel.org
Cc: stable@vger.kernel.org # v4.9
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Florian Westphal <fw@strlen.de>
---
 net/xfrm/xfrm_policy.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index ce500f847b99..e9d0df2a2ab1 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2092,12 +2092,15 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type,
 	if (unlikely(!daddr || !saddr))
 		return NULL;
 
-	rcu_read_lock();
  retry:
-	do {
-		sequence = read_seqcount_begin(&xfrm_policy_hash_generation);
-		chain = policy_hash_direct(net, daddr, saddr, family, dir);
-	} while (read_seqcount_retry(&xfrm_policy_hash_generation, sequence));
+	sequence = read_seqcount_begin(&xfrm_policy_hash_generation);
+	rcu_read_lock();
+
+	chain = policy_hash_direct(net, daddr, saddr, family, dir);
+	if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) {
+		rcu_read_unlock();
+		goto retry;
+	}
 
 	ret = NULL;
 	hlist_for_each_entry_rcu(pol, chain, bydst) {
@@ -2128,11 +2131,15 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type,
 	}
 
 skip_inexact:
-	if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence))
+	if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) {
+		rcu_read_unlock();
 		goto retry;
+	}
 
-	if (ret && !xfrm_pol_hold_rcu(ret))
+	if (ret && !xfrm_pol_hold_rcu(ret)) {
+		rcu_read_unlock();
 		goto retry;
+	}
 fail:
 	rcu_read_unlock();
 
-- 
2.26.2


             reply	other threads:[~2021-05-28 12:04 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-28 12:03 Varad Gautam [this message]
2021-05-28 15:11 ` Ahmed S. Darwish
2021-05-28 16:06   ` Varad Gautam
2021-05-28 16:04 ` [PATCH v2] " Varad Gautam
2021-05-28 16:44   ` Ahmed S. Darwish
2021-06-03  8:16     ` Steffen Klassert

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