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

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2017-09-20 at 11:04 +0100, George Dunlap wrote:
> On 09/20/2017 10:19 AM, George Dunlap wrote:
> > > diff --git a/xen/common/sched_credit2.c
> > > b/xen/common/sched_credit2.c
> > > index 32234ac..7a550db 100644
> > > --- a/xen/common/sched_credit2.c
> > > +++ b/xen/common/sched_credit2.c
> > > @@ -2923,13 +2923,13 @@ csched2_free_domdata(const struct
> > > scheduler *ops, void *data)
> > >  
> > >      write_lock_irqsave(&prv->lock, flags);
> > >  
> > > -    kill_timer(sdom->repl_timer);
> > > -    xfree(sdom->repl_timer);
> > > -
> > >      list_del_init(&sdom->sdom_elem);
> > >  
> > >      write_unlock_irqrestore(&prv->lock, flags);
> > >  
> > > +    kill_timer(sdom->repl_timer);
> > > +    xfree(sdom->repl_timer);
> > 
> > Any particular reason for moving the kill_timer() as well as the
> > xfree
> > outside the lock?  What happens if the timer goes off after the
> > irqrestore but before the kill_timer?
> 
> Looks like if the timer fires, nothing terribly bad will happen; it
> will
> just do a useless budget replenishment.
> 
It's just that it has not reason to be there, as nothing that it does
is serialized by prv->lock, so it only makes the critical section (with
IRQ disabled) longer, for no reason, which is bad, as this being a
write_lock(), it'll stop readers too.

I think it was (my) mistake to put it there in the first place.

> It looks like kill_timer() disables irqs, so it could be moved inside
> the critical section.  I'm inclined to say we should do so.  I don't
> anticipate the budget replenishment ever to need to walk the domain
> list, but should that change, this would be a bear of a bug to find.
> 
Indeed it's rather unlikely for the replenishment handler to have to
use sdom_elem to go through the list of domains.

IAC, if you're concerned about that, I'd much rather put both
kill_timer() and xfree() before the critical section, rather than
after, like in the attached patch.

Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

--- Begin Message ---

• From: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
• Date: Wed, 20 Sep 2017 13:42:51 +0200

[PATCH v2] xen: credit2: fix spinlock irq-safety violation

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>
---
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 v1:
- kill_timer() and xfree() moved above critical section, instead than below.

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 32234ac..32b0363 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -2921,11 +2921,11 @@ 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);

--- End Message ---

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