[Xen-devel] [PATCH v3] xen: credit2: fix spinlock irq-safety violation

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

In commit ad4b3e1e9df34 ("xen: credit2: implement
utilization cap") xfree() was being called (for
deallocating the budget replenishment timer, during
domain destruction) inside an IRQ disabled critical
section.

That must not happen, as it uses the mem-pool's lock,
which needs to be taken with IRQ enabled. And, in fact,
we crash (in debug builds):

(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Xen BUG at spinlock.c:47
(XEN) ****************************************

Let's, therefore, kill and deallocate the timer outside of
the critical sections, when IRQs are enabled.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: osstest service owner <osstest-admin@xxxxxxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Cc: Jan Beulich <JBeulich@xxxxxxxx>
---
This was spotted by OSSTest's flight 113562:

 http://logs.test-lab.xenproject.org/osstest/logs/113562/
 
http://logs.test-lab.xenproject.org/osstest/logs/113562/test-amd64-amd64-xl-credit2/serial-godello0.log
---
Changes from v2:
- kill_timer() stays above the critical section, while xfree() goes below it,
  to avoid having a domain in the linked list of domains with a NULL timer.

Changes from v1:
- kill_timer() and xfree() moved above critical section, instead than below.
---
 xen/common/sched_credit2.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 32234ac..015e45e 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -2921,15 +2921,15 @@ csched2_free_domdata(const struct scheduler *ops, void 
*data)
     struct csched2_dom *sdom = data;
     struct csched2_private *prv = csched2_priv(ops);
 
-    write_lock_irqsave(&prv->lock, flags);
-
     kill_timer(sdom->repl_timer);
-    xfree(sdom->repl_timer);
+
+    write_lock_irqsave(&prv->lock, flags);
 
     list_del_init(&sdom->sdom_elem);
 
     write_unlock_irqrestore(&prv->lock, flags);
 
+    xfree(sdom->repl_timer);
     xfree(data);
 }
 


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