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Infinite loop in wakeup check in Curl_multi_wait

From: Dunaj, Robert via curl-users <>
Date: Wed, 26 Feb 2020 13:51:05 +0000


Our application started to have higher cpu after libcurl upgrade from 7.66.0 to 7.68.0
It turned out that the affected thread was in infinite loop in - sread is called repeatedly there in line 1202:

      if(use_wakeup && multi->wakeup_pair[0] != CURL_SOCKET_BAD) {
        if(ufds[curlfds + extra_nfds].revents & POLLIN) {
          char buf[64];
          while(1) {
            /* the reading socket is non-blocking, try to read
               data from it until it receives an error (except EINTR).
               In normal cases it will get EAGAIN or EWOULDBLOCK
               when there is no more data, breaking the loop. */
Line 1202: if(sread(multi->wakeup_pair[0], buf, sizeof(buf)) < 0) {
              if(EINTR == SOCKERRNO)
          /* do not count the wakeup socket into the returned value */

It happens on Amazon Linux AMI 2018.03 (kernel 4.14.114-83.126.amzn1.x86_64) machine.
I attached with the gdb and buf content is not changing after each read.
So I suspect that sread returns 0 (end of file).
Shouldn't == 0 check be added there?

Kind regards
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Received on 2020-02-26