Hi, I pushed the proposed fix of setting CURLOPT_NOSIGNAL to 1. This effectively makes libcurl lose its timeout ability for synchronous DNS lookups. Asynchronous DNS lookups via the c-ares library are not effected. You backtrace shows a timeout of a synchronous DNS lookup, I think (see the Curl_resolv_timeout Curl_failf call sequence). This is how you found the problem. But setting CURLOPT_NOSIGNAL to 1 makes libcurl lose its timeout ability for synchronous DNS lookups and a call to Curl_resolv_timeout can now take longer than a given timeout or might never return at all. So we're are replacing a possible segfault with with a possibly DNS lookup that takes too long or never returns. Regards, Matthias 2012/10/2 Benjamin Wang (gendwang) <gendwang@xxxxxxxxx>: > 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@xxxxxxxxxxxxxx] > Sent: 2012年9月30日 4:20 > To: Benjamin Wang (gendwang) > Cc: libvir-list@xxxxxxxxxx; Yang Zhou (yangzho) > Subject: Re: Two core dumps are generated in multi-thread scenarios > > 2012/9/23 Benjamin Wang (gendwang) <gendwang@xxxxxxxxx>: >> 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 -- Matthias Bolte http://photron.blogspot.com -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list