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

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@googlemail.com] Sent: 2012年9月30日 4:20 To: Benjamin Wang (gendwang) Cc: libvir-list(a)redhat.com; Yang Zhou (yangzho) Subject: Re: Two core dumps are generated in multi-thread scenarios 2012/9/23 Benjamin Wang (gendwang) <gendwang(a)cisco.com&gt;: > 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