Re: spin_lock error in arch/i386/kernel/time.c on APM resume

[George Anzinger <george@mvista.com>]

Pavel Machek wrote:
> Hi!
> 
> 
>>>>I agree.  Still in all that follows, no one has addressed the apparent 
>>>>race described above.  The reason the system reported the errors that 
>>>>started this thread is that the APM restore code was trying to read the 
>>>>cmos clock (I assume to set the xtime clock) WHILE the timer interrupt 
>>>>code what trying to set the cmos clock from xtime.  In other words, it is 
>>>>destroying the time it is trying to read.  I repeat "Possibly the APM 
>>>>code should change time_status to STA_UNSYNC on the way into the sleep."  
>>>>I am not sure how ntp is supposed to react to the resume but I suspect 
>>>>that the system time is rather out of sync...
>>>
>>>
>>>It needs to work without NTP, too. You don't get NTP on plane (etc)
>>>where suspend is most usefull.
>>>
>>>We have CMOS clock, it should be possible to get time from there
>>>without resorting to NTP..
>>
>>Eh... sure, but... the bug was reported because the system was attempting 
>>to update the cmos clock (which it does every ~11 min.) during APM exit.   
>>It does this IF AND ONLY IF the system is synced to an external source as 
>>indicated by the STA_UNSYNC bit being cleared in the time_state.  Now, I 
>>don't know what or how APM and NTP are supposed to play together, but I 
>>suspect that on entry to APM time is no longer synced, thus my comment.
>>
>>As to your comment, the bug would never have shown its ugly face if the 
>>system wasn't using NTP.
> 
> 
> Uh, ok, you are right. We should set time to STA_UNSYNC so that we do
> not write back to CMOS during/shortly after resume. I did not realize
> what STA_UNSYNC means. Perhaps you have patch to  do that somewhere?
> ;-))))

Zwane has convinced me that the real problem is doing the right thing (tm) in 
the APM code, i.e. not allowing the timer interrupt until after reading the cmos 
clock, for which he has a patch tendered.

-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/