This is a note to let you know that I've just added the patch titled xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: xfrm-policy-read-seqcount-outside-of-rcu-read-side-in-xfrm_policy_lookup_bytype.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From d7b0408934c749f546b01f2b33d07421a49b6f3e Mon Sep 17 00:00:00 2001 From: Varad Gautam <varad.gautam@xxxxxxxx> Date: Fri, 28 May 2021 18:04:06 +0200 Subject: xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype From: Varad Gautam <varad.gautam@xxxxxxxx> commit d7b0408934c749f546b01f2b33d07421a49b6f3e upstream. 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: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx> Cc: linux-rt-users <linux-rt-users@xxxxxxxxxxxxxxx> Cc: netdev@xxxxxxxxxxxxxxx Cc: stable@xxxxxxxxxxxxxxx # v4.9 Cc: Steffen Klassert <steffen.klassert@xxxxxxxxxxx> Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> Cc: Jakub Kicinski <kuba@xxxxxxxxxx> Cc: Florian Westphal <fw@xxxxxxxxx> Cc: "Ahmed S. Darwish" <a.darwish@xxxxxxxxxxxxx> Signed-off-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx> Acked-by: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/xfrm/xfrm_policy.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) --- a/net/xfrm/xfrm_policy.c +++ b/net/xfrm/xfrm_policy.c @@ -2089,12 +2089,15 @@ static struct xfrm_policy *xfrm_policy_l 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) { @@ -2125,11 +2128,15 @@ static struct xfrm_policy *xfrm_policy_l } 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(); Patches currently in stable-queue which might be from varad.gautam@xxxxxxxx are queue-5.4/xfrm-policy-read-seqcount-outside-of-rcu-read-side-in-xfrm_policy_lookup_bytype.patch