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Re: [libvirt] [PATCH] util: make it more robust to calculate timeout value



On 2015/5/15 19:09, Michal Privoznik wrote:

> On 15.05.2015 08:26, zhang bo wrote:
>> When we change system clock to years ago, a certain CPU may use up 100% cputime.
>> The reason is that in function virEventPollCalculateTimeout(), we assign the 
>> unsigned long long result to an INT variable, 
>>         *timeout = then - now; // timeout is INT, and then/now are long long
>>         if (*timeout < 0)
>>             *timeout = 0;
>> there's a chance that variable @then minus variable @now may be a very large number 
>> that overflows INT value expression, then *timeout will be negative and be assigned to 0.
>> Next the 'poll' in function virEventPollRunOnce() will get into an 'endless' while loop there.
>> thus, the cpu that virEventPollRunOnce() thread runs on will go up to 100%. 
>>
>> Although as we discussed before in https://www.redhat.com/archives/libvir-list/2015-May/msg00400.html
>> it should be prohibited to set-time while other applications are running, but it does 
>> seems to have no harm to make the codes more robust.
>>
>> Signed-off-by: Wang Yufei <james wangyufei huawei com>
>> Signed-off-by: Zhang Bo <oscar zhangbo huawei com>
>> ---
>>  src/util/vireventpoll.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c
>> index ffda206..5f5a149 100644
>> --- a/src/util/vireventpoll.c
>> +++ b/src/util/vireventpoll.c
>> @@ -357,9 +357,10 @@ static int virEventPollCalculateTimeout(int *timeout)
>>              return -1;
>>  
>>          EVENT_DEBUG("Schedule timeout then=%llu now=%llu", then, now);
>> -        *timeout = then - now;
>> -        if (*timeout < 0)
>> +        if (then < now)
>>              *timeout = 0;
>> +        else
>> +            *timeout = (then - now) & 0x7FFFFFFF;
> 
> You're trying to make this an unsigned value. What's wrong with plain
> typecast?


No, I'm trying to give an positive value that an int variable can express,


> 
>>      } else {
>>          *timeout = -1;
>>      }
>>
> 
> I must say this is ugly. If the system clock is changed, all the
> timeouts should fire, shouldn't they? Otherwise important events can be
> missed.
> 

In fact, the events will not be handled.

In virEventPollDispatchTimeouts

        if (eventLoop.timeouts[i].expiresAt <= (now+20)) {
            ...
            (cb)(timer, opaque);

'expiresAt' is much bigger than 'now' because the system clock has been changed to long long ago.

Assign an unsigned long long value to an int value is out of control, we don't know whether it be negative
or positive because of overflow of int. I know this patch can not fix system clock changed problem, but at
least, the 'timeout' will be under control, and the cpu up to 100% situation can be fixed, that's my point.



> Michal
> 
> .
> 




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