Re: Re: [PATCH] [PATCH] mm: disable preemption before swapcache_free

["zhaowuyun@wingtech.com" <zhaowuyun@wingtech.com>]

>On Wed 25-07-18 17:53:07, zhaowuyun@wingtech.com wrote:
>> >[Please do not top post - thank you]
>> >[CC Hugh - the original patch was http://lkml.kernel.org/r/2018072514375722198958@wingtech.com]
>> >
>> >On Wed 25-07-18 15:57:55, zhaowuyun@wingtech.com wrote:
>> >> That is a BUG we found in mm/vmscan.c at KERNEL VERSION 4.9.82
>> >
>> >The code is quite similar in the current tree as well.
>> >
>> >> Sumary is TASK A (normal priority) doing __remove_mapping page preempted by TASK B (RT priority) doing __read_swap_cache_async,
>> >> the TASK A preempted before swapcache_free, left SWAP_HAS_CACHE flag in the swap cache,
>> >> the TASK B which doing __read_swap_cache_async, will not success at swapcache_prepare(entry) because the swap cache was exist, then it will loop forever because it is a RT thread...
>> >> the spin lock unlocked before swapcache_free, so disable preemption until swapcache_free executed ...
>> >
>> >OK, I see your point now. I have missed the lock is dropped before
>> >swapcache_free. How can preemption disabling prevent this race to happen
>> >while the code is preempted by an IRQ?
>> >--
>> >Michal Hocko
>> >SUSE Labs
>>
>> Hi Michal,
>>
>> The action what processes __read_swap_cache_async is on the process context, so I think disable preemption is enough.
>
>So what you are saying is that no IRQ or other non-process contexts will
>not loop in __read_swap_cache_async so the live lock is not possible?
>--
>Michal Hocko
>SUSE Labs 


I think that __read_swap_cache_async will not running under IRQ contexts. 
If running under other non-process contexts, I think it must at the other CPU, will not encounter this dead loop.

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zhaowuyun@wingtech.com