| Message ID | 20210528120357.29542-1-varad.gautam@suse.com |
|---|---|
| State | New |
| Headers | show |
| Series | xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype | expand |
On 5/28/21 5:11 PM, Ahmed S. Darwish wrote: > 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. > Thanks, corrected it in v2. > Kind regards, > > -- > Ahmed S. Darwish > Linutronix GmbH >
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();
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(-)