On Sun, 2017-10-08 at 18:18 +0300, Ido Schimmel wrote:
> Without the rwlock and with PREEMPT_RCU we're no longer guaranteed to be
> in non-preemptible context when performing a route lookup, so use
> raw_cpu_ptr() instead.
>
> Takes care of the following splat:
> [ 122.221814] BUG: using smp_processor_id() in preemptible [00000000] code:
> sshd/2672
> [ 122.221845] caller is debug_smp_processor_id+0x17/0x20
> [ 122.221866] CPU: 0 PID: 2672 Comm: sshd Not tainted
> 4.14.0-rc3-idosch-next-custom #639
> [ 122.221880] Hardware name: Mellanox Technologies Ltd.
> MSN2100-CB2FO/SA001017, BIOS 5.6.5 06/07/2016
> [ 122.221893] Call Trace:
> [ 122.221919] dump_stack+0xb1/0x10c
> [ 122.221946] ? _atomic_dec_and_lock+0x124/0x124
> [ 122.221974] ? ___ratelimit+0xfe/0x240
> [ 122.222020] check_preemption_disabled+0x173/0x1b0
> [ 122.222060] debug_smp_processor_id+0x17/0x20
> [ 122.222083] ip6_pol_route+0x1482/0x24a0
> ...
>
> Fixes: 66f5d6ce53e6 ("ipv6: replace rwlock with rcu and spinlock in
> fib6_table")
> Signed-off-by: Ido Schimmel <ido...@mellanox.com>
> ---
Thanks Ido for this patch.
IMO, we no longer play this read_lock() -> write_lock() game since
ip6_dst_gc() could be called from rt6_make_pcpu_route()
So we might simplify things quite a bit, by blocking BH (and thus
preventing preemption)
Something like :
net/ipv6/route.c | 26 ++++++--------------------
1 file changed, 6 insertions(+), 20 deletions(-)
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index
399d1bceec4a6e6736c367e706dd2acbd4093d58..606e80325b21c0e10a02e9c7d5b3fcfbfc26a003
100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1136,15 +1136,7 @@ static struct rt6_info *rt6_make_pcpu_route(struct
rt6_info *rt)
dst_hold(&pcpu_rt->dst);
p = this_cpu_ptr(rt->rt6i_pcpu);
prev = cmpxchg(p, NULL, pcpu_rt);
- if (prev) {
- /* If someone did it before us, return prev instead */
- /* release refcnt taken by ip6_rt_pcpu_alloc() */
- dst_release_immediate(&pcpu_rt->dst);
- /* release refcnt taken by above dst_hold() */
- dst_release_immediate(&pcpu_rt->dst);
- dst_hold(&prev->dst);
- pcpu_rt = prev;
- }
+ BUG_ON(prev);
rt6_dst_from_metrics_check(pcpu_rt);
return pcpu_rt;
@@ -1739,31 +1731,25 @@ struct rt6_info *ip6_pol_route(struct net *net, struct
fib6_table *table,
struct rt6_info *pcpu_rt;
dst_use_noref(&rt->dst, jiffies);
+ local_bh_disable();
pcpu_rt = rt6_get_pcpu_route(rt);
- if (pcpu_rt) {
- rcu_read_unlock();
- } else {
+ if (!pcpu_rt) {
/* atomic_inc_not_zero() is needed when using rcu */
if (atomic_inc_not_zero(&rt->rt6i_ref)) {
- /* We have to do the read_unlock first
- * because rt6_make_pcpu_route() may trigger
- * ip6_dst_gc() which will take the write_lock.
- *
- * No dst_hold() on rt is needed because
grabbing
+ /* No dst_hold() on rt is needed because
grabbing
* rt->rt6i_ref makes sure rt can't be released.
*/
- rcu_read_unlock();
pcpu_rt = rt6_make_pcpu_route(rt);
rt6_release(rt);
} else {
/* rt is already removed from tree */
- rcu_read_unlock();
pcpu_rt = net->ipv6.ip6_null_entry;
dst_hold(&pcpu_rt->dst);
}
}
-
+ local_bh_enable();
+ rcu_read_unlock();
trace_fib6_table_lookup(net, pcpu_rt, table->tb6_id, fl6);
return pcpu_rt;
}
