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

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

>On Wed, 25 Jul 2018 14:37:58 +0800 "zhaowuyun@wingtech.com" <zhaowuyun@wingtech.com> wrote:
>
>> From: zhaowuyun <zhaowuyun@wingtech.com>
>> 
>> issue is that there are two processes A and B, A is kworker/u16:8
>> normal priority, B is AudioTrack, RT priority, they are on the
>> same CPU 3.
>> 
>> The task A preempted by task B in the moment
>> after __delete_from_swap_cache(page) and before swapcache_free(swap).
>> 
>> The task B does __read_swap_cache_async in the do {} while loop, it
>> will never find the page from swapper_space because the page is removed
>> by the task A, and it will never sucessfully in swapcache_prepare because
>> the entry is EEXIST.
>> 
>> The task B then stuck in the loop infinitely because it is a RT task,
>> no one can preempt it.
>> 
>> so need to disable preemption until the swapcache_free executed.
>
>Yes, right, sorry, I must have merged cbab0e4eec299 in my sleep.
>cond_resched() is a no-op in the presence of realtime policy threads
>and using to attempt to yield to a different thread it in this fashion
>is broken.
>
>Disabling preemption on the other side of the race should fix things,
>but it's using a bandaid to plug the leakage from the earlier bandaid.
>The proper way to coordinate threads is to use a sleeping lock, such
>as a mutex, or some other wait/wakeup mechanism.
>
>And once that's done, we can hopefully eliminate the do loop from
>__read_swap_cache_async().  That also services ENOMEM from
>radix_tree_insert(), but __add_to_swap_cache() appears to handle that
>OK and we shouldn't just loop around retrying the insert and the
>radix_tree_preload() should ensure that radix_tree_insert() never fails
>anyway.  Unless we're calling __read_swap_cache_async() with screwy
>gfp_flags from somewhere.
>
> 


Your are right, it is a bandaid ...
Could you provide some suggestion more specific about how to use sleeping lock/some other wait/wakeup mechanism to fix this issue? Thanks very much!
Our project really needs a fix to this issue ...


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zhaowuyun@wingtech.com