FAILED: patch "[PATCH] xfrm: policy: Read seqcount outside of rcu-read side in" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d7b0408934c749f546b01f2b33d07421a49b6f3e Mon Sep 17 00:00:00 2001
From: Varad Gautam <[email protected]>
Date: Fri, 28 May 2021 18:04:06 +0200
Subject: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in
xfrm_policy_lookup_bytype

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.

index b74f28cabe24..8c56e3e59c3c 100644
@@ -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();