[libvirt] Two core dumps are generated in multi-thread scenarios

[Benjamin Wang (gendwang)]

Hi Matthias,
   This can't be reproduced 100%. I reproduce this case twice. But when I set the CURLOPT_NOSIGNAL to 1. I didn't find the similar
core again. And it seems that everything works well. What do you mean " stuck in a DNS lookup"? 

B.R.
Benjamin Wang

-----Original Message-----
From: Matthias Bolte [mailto:matthias.bolte at googlemail.com] 
Sent: 2012年9月30日 4:20
To: Benjamin Wang (gendwang)
Cc: libvir-list at redhat.com; Yang Zhou (yangzho)
Subject: Re: Two core dumps are generated in multi-thread scenarios

2012/9/23 Benjamin Wang (gendwang) <gendwang at cisco.com>:
> Hi,
>   I found two core dumps generated in multi-thread scenarios in ESX part.
>
> Case1: libcurl support multi-thread
> core dump:
> #12 0x00002aaabea89712 in addbyter () from /usr/local/lib/libcurl.so.4
> #13 0x00002aaabea89b86 in dprintf_formatf () from
> /usr/local/lib/libcurl.so.4
> #14 0x00002aaabea8b055 in curl_mvsnprintf () from
> /usr/local/lib/libcurl.so.4
> #15 0x00002aaabea7678f in Curl_failf () from 
> /usr/local/lib/libcurl.so.4
> #16 0x00002aaabea6d871 in Curl_resolv_timeout () from
> /usr/local/lib/libcurl.so.4
> #17 0x00000006e8a8f230 in ?? ()
>
> Fix code:
> esxVI_CURL_Connect() in esx_vi.c:
> I add a new line as following:
> curl_easy_setopt(curl->handle, CURLOPT_NOSIGNAL, 1);

It took me a moment reading libcurl code until I figured out what might be happening here. The problem is that Curl_resolv_timeout uses SIGALRM + sigsetjmp/siglongjmp to realize the timeout logic. This implementation is not thread-safe as the SIGALRM might be executed on a different thread than the original thread that started the call to Curl_resolv_timeout. This in turn results in the call to Curl_resolv_timeout being continued via siglongjmp (called from the SIGALRM handler) on different thread. Setting CURLOPT_NOSIGNAL to 1 makes libcurl avoid the SIGALRM + sigsetjmp/siglongjmp implementation.
This solves the problem but with the cost of losing the timeout capability.

In your case a DNS lookup took longer than libcurl was willing to wait and a timeout aborted it. But the call to Curl_failf (as part of the timeout error handling) was made on the wrong thread (I think) making it segfault. IMHO there is no ideal solution here, because with CURLOPT_NOSIGNAL set to 0 (the default) libcurl can realize DNS lookup with timeout, but the error handling might occur on the wrong thread.
But with CURLOPT_NOSIGNAL set to 1 the segfault is avoided but libcurl might get stuck in a DNS lookup.

Are you able to reproduce this problem and can you confirm that setting CURLOPT_NOSIGNAL to 1 fixes it?

--
Matthias Bolte
http://photron.blogspot.com