Re: [PATCH] ftrace: Disable function tracing during suspend/resumeand hibernation, again

[Srivatsa S. Bhat]

On 05/22/2012 03:46 AM, Steven Rostedt wrote:

> On Mon, 2012-05-21 at 23:55 +0200, Rafael J. Wysocki wrote:
>> On Monday, May 21, 2012, Srivatsa S. Bhat wrote:
>>> If function tracing is enabled for some of the low-level suspend/resume
>>> functions, it leads to triple fault during resume from suspend, ultimately
>>> ending up in a reboot instead of a resume (or a total refusal to come out
>>> of suspended state, on some machines).
>>>
>>> This issue was explained in more detail in commit f42ac38c59e0a03d (ftrace:
>>> disable tracing for suspend to ram). However, the changes made by that commit
>>> got reverted by commit cbe2f5a6e84eebb (tracing: allow tracing of
>>> suspend/resume & hibernation code again). So, unfortunately since things are
>>> not yet robust enough to allow tracing of low-level suspend/resume functions,
>>> suspend/resume is still broken when ftrace is enabled.
>>>
>>> So fix this by disabling function tracing during suspend/resume & hibernation.
>>
>> Steve, any comments?
> 
> I was actually the one to recommend this solution ;-)
> 


And I am grateful to you for that :-)

> But that said, I actually do have some.
> 
>>
>>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> ---
>>>
>>>  kernel/power/hibernate.c |    6 ++++++
>>>  kernel/power/suspend.c   |    3 +++
>>>  2 files changed, 9 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
>>> index e09dfbf..52a1817 100644
>>> --- a/kernel/power/hibernate.c
>>> +++ b/kernel/power/hibernate.c
>>> @@ -352,6 +352,7 @@ int hibernation_snapshot(int platform_mode)
>>>  	}
>>>  
>>>  	suspend_console();
>>> +	ftrace_stop();
> 
> 
> Is this a point of no return?
> 


Yes, especially as far as userspace is concerned.

> This probably isn't a problem, but I'm being a little paranoid. As
> ftrace_stop/start() is just an on off switch, anyplace that calls
> ftrace_start() will enable function tracing again. Ftrace_start() is
> called by tracing_start() which is called when
> the /sys/kernel/debug/tracing/trace file is opened and then closed. If
> the close happens while the suspend is starting, it possibly can start
> function tracing again.
> 
> But this race would have to be done by the user, which should have full
> control as they are in the process of suspending the machine. But they
> may do it by accident.
> 


Luckily, at all these places, all the userspace processes (and the freezable
kernel threads) are already frozen. So nobody will be running at this point
to open/close the trace file.

So there is no problem right?

(Of course, the non-freezable kernel threads might be running at this point,
but IIUC, they can't trigger ftrace_start() from some other internal path..)

Regards,
Srivatsa S. Bhat

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