Xen devel - Apr 2008 - [PATCH] Fix deadlock in schedule.c at TRACE mode

Hi,

In schedule.c, schedule() and sched_adjust() call trace functions during
acquiring lock of schedule_lock in each cpu''s schedule_data. When trace
buffers are enabled, the trace function (__trace_var()) may call
vcpu_wake() by calling send_guest_global_virq(). In the case, a deadlock
occurs when acquiring lock of schedule_lock.

Attached patch fixes this problem.

Signed-off-by: Naoki Nishiguchi <nisiguti@jp.fujitsu.com>

Regards,
Naoki Nishiguchi


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Hi, Naoki

I have two questions about this.

1)How to reproduce your deadlock ?
  Would you give me your test environment to reproduce this deadlock?
  Is it easily reproduced by running xenmon.py or xentrace 
  with one or two guest domain(s)?
　or Any additional condition needed?

2)About fixing code,
  I think __trace_var() should fix for this issue not schedule()
  This issue cannot be fixed by modify the __trace_var()?

Thanks
Atsushi SAKAI


NISHIGUCHI Naoki <nisiguti@jp.fujitsu.com> wrote:
> Hi,
> 
> In schedule.c, schedule() and sched_adjust() call trace functions during
> acquiring lock of schedule_lock in each cpu''s schedule_data. When
trace
> buffers are enabled, the trace function (__trace_var()) may call
> vcpu_wake() by calling send_guest_global_virq(). In the case, a deadlock
> occurs when acquiring lock of schedule_lock.
> 
> Attached patch fixes this problem.
> 
> Signed-off-by: Naoki Nishiguchi <nisiguti@jp.fujitsu.com>
> 
> Regards,
> Naoki Nishiguchi


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Hi, Atsushi

Atsushi SAKAI wrote:
> I have two questions about this.
> 
> 1)How to reproduce your deadlock ?
>   Would you give me your test environment to reproduce this deadlock?
>   Is it easily reproduced by running xenmon.py or xentrace 
>   with one or two guest domain(s)?
> 　or Any additional condition needed?
This deadlock can be easily reproduced by running xenmon.py without
guest domain.
Furthermore, this deadlock occurs easier by applying my patch to xenbaked.c.

Subject of my patch is "[PATCH] Fix access to trace buffer after
xentrace changes".
> 2)About fixing code,
>   I think __trace_var() should fix for this issue not schedule()
>   This issue cannot be fixed by modify the __trace_var()?
Thanks for your advise.
I agree with you.
I fixed this deadlock using tasklet in trace.c.

Here is the patch.

Thanks,
Naoki Nishiguchi


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On 24/4/08 08:03, "NISHIGUCHI Naoki" <nisiguti@jp.fujitsu.com>
wrote:
>> 2)About fixing code,
>>   I think __trace_var() should fix for this issue not schedule()
>>   This issue cannot be fixed by modify the __trace_var()?
> 
> Thanks for your advise.
> I agree with you.
> I fixed this deadlock using tasklet in trace.c.
> 
> Here is the patch.
Thanks. This was my bad as I removed the old softirq and couldn''t see a
reason not to call wake() directly. I''ll add a comment when I apply
your
patch.

 -- Keir



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