Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator

[Paolo Valente]

> Il giorno 04 apr 2017, alle ore 17:28, Bart Van Assche <bart.vanassche@xxxxxxxxxxx> ha scritto:
> 
> On Tue, 2017-04-04 at 12:42 +0200, Paolo Valente wrote:
>>> Il giorno 31 mar 2017, alle ore 17:31, Bart Van Assche <bart.vanassche@xxxxxxxxxxx> ha scritto:
>>> 
>>> On Fri, 2017-03-31 at 14:47 +0200, Paolo Valente wrote:
>>>> +               delta_ktime = ktime_get();
>>>> +       delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
>>>> +       delta_usecs = ktime_to_us(delta_ktime);
>>> 
>>> This patch changes the type of the variable in which the result of ktime_to_us()
>>> is stored from u64 into u32 and next compares that result with LONG_MAX. Since
>>> ktime_to_us() returns a signed 64-bit number, are you sure you want to store that
>>> result in a 32-bit variable? If ktime_to_us() would e.g. return 0xffffffff00000100
>>> or 0x100000100 then the assignment will truncate these numbers to 0x100.
>> 
>> The instruction above the assignment you highlight stores in
>> delta_ktime the difference between 'now' and the last budget start.
>> The latter may have happened at most about 100 ms before 'now'.  So
>> there should be no overflow issue.
> 
> Hello Paolo,
> 
> Please double check the following code: if (delta_usecs < 1000 || delta_usecs >= LONG_MAX)
> Since delta_usecs is a 32-bit variable and LONG_MAX a 64-bit constant on 64-bit systems
> I'm not sure that code will do what it is intended to do.
> 

Yes, sorry. Actually, it never occurred to me to see that extra condition to hold over the last eight years on 32-bit systems. So I think I will just remove it. Unless Fabio, who inserted that condition several years ago, has something to say.

Thanks,
Paolo

> Thanks,
> 
> Bart.