Furthermore, I tried to disable the crawler with the '- no_lru_crawler'
command parameter, and it gives the same error. I wonder why it does not
disable the crawler lru as it supposes to do.
On Monday, May 31, 2021 at 1:02:38 AM UTC-4 Qingchen Dang wrote:
> Hi,
>
> I am implementing a framework based on Memcached. There's a problem that
> confused me a lot. The framework basically change the eviction policy, so
> when it calls to evict an item, it might not evict the tail item at COLD
> LRU, instead it will look for a "more suitable" item to evict and it will
> reinsert the tail items to the head of COLD queue.
>
> It mostly works fine, but sometimes it causes a SegFault when reinsertion
> happens very frequently (like in almost each eviction). The SegFault is
> triggered in the crawler part. As attached, it seems when the crawler loops
> through the item queue, it reaches an invalid memory address. The bug
> happens after around 50000000~10000000 GET/SET (9:1) operations. I used
> Memaslap for testing.
>
> Could anyone give me some suggestions of the reasons which cause such
> error?
>
> Here is the gdb messages:
>
> *Thread 8 "memcached" received signal SIGSEGV, Segmentation fault.*
>
> *[Switching to Thread 0x7ffff4d6c700 (LWP 36414)]*
>
> *do_item_crawl_q (it=it@entry=0x55555579e7e0 <crawlers+12320>)*
>
> * at items.c:2015*
>
> *2015 it->prev->next = it->next;*
>
> *(gdb) print it->prev*
>
> *$5 = (struct _stritem *) 0x4f4d6355616d5471*
>
> *(gdb) print it->prev->next*
>
> *Cannot access memory at address 0x4f4d6355616d5479*
>
> *(gdb) print it->next*
>
> *$6 = (struct _stritem *) 0x7a59324376753351*
>
> *(gdb) print it->next->prev*
>
> *Cannot access memory at address 0x7a59324376753361*
>
> *(gdb) print it->nkey*
>
> *$7 = 0 '\000'*
>
> *(gdb) *
> Here is the part that triggers the error:
>
> *2012 assert(it->next != it);*
>
> *2013 if (it->next) {*
>
> *2014 assert(it->prev->next == it);*
>
> *2015 it->prev->next = it->next;*
>
> *2016 it->next->prev = it->prev;*
>
> *2017 } else {*
>
> *2018 /* Tail. Move this above? */*
>
> *2019 it->prev->next = 0;*
>
> *2020 }*
>
> (I'm also confused why the assert function in line 2014 does not give
> error?)
>
> Thank you very much for helping!
>
> Best,
>
> Qingchen
>
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