Messages in this thread | | | Date | Tue, 28 Apr 2020 21:22:30 +0000 | From | Wei Yang <> | Subject | Re: [PATCH v2] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots() |
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On Mon, Apr 27, 2020 at 08:55:33AM +0800, Huang, Ying wrote: >Wei Yang <richard.weiyang@gmail.com> writes: > >> On Sun, Apr 26, 2020 at 09:07:11AM +0800, Huang, Ying wrote: >>>Wei Yang <richard.weiyang@gmail.com> writes: >>> >>>> On Fri, Apr 24, 2020 at 10:02:58AM +0800, Huang, Ying wrote: >>>>>Wei Yang <richard.weiyang@gmail.com> writes: >>>>> >>>> [...] >>>>>>> >>>>>>>if "offset > si->highest_bit" is true and "offset < scan_base" is true, >>>>>>>scan_base need to be returned. >>>>>>> >>>>>> >>>>>> When this case would happen in the original code? >>>>> >>>>>In the original code, the loop can still stop. >>>>> >>>> >>>> Sorry, I don't get your point yet. >>>> >>>> In original code, there are two separate loops >>>> >>>> while (++offset <= si->highest_bit) { >>>> } >>>> >>>> while (offset < scan_base) { >>>> } >>>> >>>> And for your condition, (offset > highest_bit) && (offset < scan_base), which >>>> terminates the first loop and fits the second loop well. >>>> >>>> Not sure how this condition would stop the loop in original code? >>> >>>Per my understanding, in your code, if some other task changes >>>si->highest_bit to be less than scan_base in parallel. The loop may >>>cannot stop. >> >> When (offset > scan_base), (offset > si->highest_bit) means offset will be >> set to si->lowest_bit. >> >> When (offset < scan_base), next_offset() would always increase offset till >> offset is scan_base. >> >> Sorry, I didn't catch your case. Would you minding giving more detail? > >Don't think in single thread model. There's no lock to prevent other >tasks to change si->highest_bit simultaneously. For example, task B may >change si->highest_bit to be less than scan_base in task A. >
Yes, I am trying to think about it in parallel mode.
Here are the cases, it might happen in parallel when task B change highest_bit to be less than scan_base.
(1) offset v +-------------------+------------------+ ^ ^ ^ lowest_bit highest_bit scan_base
(2) offset v +-------------------+------------------+ ^ ^ ^ lowest_bit highest_bit scan_base
(3) offset v +-------------------+------------------+ ^ ^ ^ lowest_bit highest_bit scan_base
Case (1), (offset > highest) && (offset > scan_base), offset would be set to lowest_bit. This looks good.
Case (2), (offset > highest) && (offset < scan_base), since offset is less than scan_base, it wouldn't be set to lowest. Instead it will continue to scan_base.
Case (3), almost the same as Case (2).
In Case (2) and (3), one thing interesting is the loop won't stop at highest_bit, while the behavior is the same as original code.
Maybe your concern is this one? I still not figure out your point about the infinite loop. Hope you would share some light on it.
>Best Regards, >Huang, Ying > >>> >>>Best Regards, >>>Huang, Ying >>> >>>>>Best Regards, >>>>>Huang, Ying >>>>> >>>>>>>Again, the new code doesn't make it easier to find this kind of issues. >>>>>>> >>>>>>>Best Regards, >>>>>>>Huang, Ying
-- Wei Yang Help you, Help me
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