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();
