[PATCH]sched_credit: Hold lock while dump scheduler info (RE: [Xen-devel] dump runq with debug key 'r' may cause dead loop)

["Wei, Gang" <gang.wei@xxxxxxxxx>]

Here is the patch.

sched_credit: Hold lock while dump scheduler info

Dump runq with debug key 'r' may cause dead loop like below:

(XEN) active vcpus:
(XEN)     1: [1.0] pri=0 flags=0 cpu=0 credit=263 [w=256]
(XEN)     2: [0.2] pri=0 flags=0 cpu=5 credit=284 [w=256]
(XEN)     3: [0.2] pri=0 flags=0 cpu=5 credit=282 [w=256]
...
(XEN)  xxxxx: [0.2] pri=0 flags=0 cpu=2 credit=54 [w=256]
...
(XEN)  xxxxx: [0.2] pri=0 flags=0 cpu=3 credit=-48 [w=256]
...

This means the active vcpu 0.2 became non-active with the active list element 
empty just after it was accessed in the loop '2:'.

We should always hold a lock before access scheduler related list, even in the 
debug purpose dump code.

Signed-off-by: Wei Gang <gang.wei@xxxxxxxxx>

diff -r 6241fa0ad1a9 xen/common/sched_credit.c
--- a/xen/common/sched_credit.c Thu Mar 03 18:52:09 2011 +0000
+++ b/xen/common/sched_credit.c Sun Mar 06 04:31:57 2011 +0800
@@ -1452,6 +1452,10 @@ csched_dump(const struct scheduler *ops)
     struct list_head *iter_sdom, *iter_svc;
     struct csched_private *prv = CSCHED_PRIV(ops);
     int loop;
+    unsigned long flags;
+
+    spin_lock_irqsave(&(prv->lock), flags);
+
 #define idlers_buf keyhandler_scratch
 
     printk("info:\n"
@@ -1500,6 +1504,8 @@ csched_dump(const struct scheduler *ops)
         }
     }
 #undef idlers_buf
+
+    spin_unlock_irqrestore(&(prv->lock), flags);
 }
 
 static int

Keir Fraser wrote on 2011-03-04:
> On 04/03/2011 09:40, "Wei, Gang" <gang.wei@xxxxxxxxx> wrote:
> 
>> Recently I found dump runq with debug key 'r' may cause dead loop like
>> below:
>> 
>> (XEN) active vcpus:
>> (XEN)    1: [1.0] pri=0 flags=0 cpu=0 credit=263 [w=256]
>> (XEN)    2: [0.2] pri=0 flags=0 cpu=5 credit=284 [w=256]
>> (XEN)    3: [0.2] pri=0 flags=0 cpu=5 credit=282 [w=256]
>> ...
>> (XEN)  xxxxx: [0.2] pri=0 flags=0 cpu=2 credit=54 [w=256] ...
>> (XEN)  xxxxx: [0.2] pri=0 flags=0 cpu=3 credit=-48 [w=256] ...
>> 
>> This means the active vcpu 0.2 became non-active just after it was
>> access in the loop '2:', and that list element became empty state
>> (head->next==next).
>> 
>> Should we always hold a lock before access any schedule related
>> list, even in the debug purpose dump code? If it is not acceptable,
>> then we'd better add a
>> list_empty() check in the dump functions which access schedule
>> related list at least to avoid such a dead loop.
> 
> The appropriate lock should be taken. Please send a patch.

Jimmy

Attachment: csched_dump_fix.patch
Description: csched_dump_fix.patch

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