| Message ID | 20210528160407.32127-1-varad.gautam@suse.com (mailing list archive) |
|---|---|
| State | Awaiting Upstream |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [v2] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype | expand |
| Context | Check | Description |
|---|---|---|
| netdev/cover_letter | success | Link |
| netdev/fixes_present | success | Link |
| netdev/patch_count | success | Link |
| netdev/tree_selection | success | Guessed tree name to be net-next |
| netdev/subject_prefix | warning | Target tree name not specified in the subject |
| netdev/cc_maintainers | success | CCed 7 of 7 maintainers |
| netdev/source_inline | success | Was 0 now: 0 |
| netdev/verify_signedoff | success | Link |
| netdev/module_param | success | Was 0 now: 0 |
| netdev/build_32bit | success | Errors and warnings before: 11 this patch: 11 |
| netdev/kdoc | success | Errors and warnings before: 0 this patch: 0 |
| netdev/verify_fixes | success | Link |
| netdev/checkpatch | warning | WARNING: 'Thead' may be misspelled - perhaps 'Thread'? |
| netdev/build_allmodconfig_warn | success | Errors and warnings before: 11 this patch: 11 |
| netdev/header_inline | success | Link |
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: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") > Signed-off-by: Varad Gautam <varad.gautam@suse.com> Acked-by: Ahmed S. Darwish <a.darwish@linutronix.de>
On Fri, May 28, 2021 at 06:44:37PM +0200, 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: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") > > Signed-off-by: Varad Gautam <varad.gautam@suse.com> > > Acked-by: Ahmed S. Darwish <a.darwish@linutronix.de> Applied, thanks a lot!
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: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") 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> Cc: "Ahmed S. Darwish" <a.darwish@linutronix.de> --- v2: Correct 'Fixes:' to the right commit. net/xfrm/xfrm_policy.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)