net_sched: qdisc_alloc_handle() can be too slow

When trying to allocate ~32768 qdiscs using autohandle mechanism, we can
fill the space managed by kernel (handles in [8000-FFFF]:0000 range)

But O(N^2) qdisc_alloc_handle() loops 0x10000 times instead of 0x8000

time tc add qdisc add dev eth0 parent 10:7fff pfifo limit 10
RTNETLINK answers: Cannot allocate memory
real    1m54.826s
user    0m0.000s
sys     0m0.004s

INFO: rcu_sched_state detected stall on CPU 0 (t=60000 jiffies)

Half number of loops, and add a cond_resched() call.
We hold rtnl at this point.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Dave Taht <dave.taht@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 5 deletions

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index dca6c1a576f7..3d8981fde301 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -618,20 +618,24 @@ void qdisc_class_hash_remove(struct Qdisc_class_hash *clhash,
 }
 EXPORT_SYMBOL(qdisc_class_hash_remove);
 
-/* Allocate an unique handle from space managed by kernel */
-
+/* Allocate an unique handle from space managed by kernel
+ * Possible range is [8000-FFFF]:0000 (0x8000 values)
+ */
 static u32 qdisc_alloc_handle(struct net_device *dev)
 {
-        int i = 0x10000;
+        int i = 0x8000;
         static u32 autohandle = TC_H_MAKE(0x80000000U, 0);
 
         do {
                 autohandle += TC_H_MAKE(0x10000U, 0);
                 if (autohandle == TC_H_MAKE(TC_H_ROOT, 0))
                         autohandle = TC_H_MAKE(0x80000000U, 0);
-        } while (qdisc_lookup(dev, autohandle) && --i > 0);
+                if (!qdisc_lookup(dev, autohandle))
+                        return autohandle;
+                cond_resched();
+        } while (--i > 0);
 
-        return i > 0 ? autohandle : 0;
+        return 0;
 }
 
 void qdisc_tree_decrease_qlen(struct Qdisc *sch, unsigned int n)