[Bug 8656] New: Flood of syslog() calls temporarily blocks process

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=8656

            Bug ID: 8656
           Summary: Flood of syslog() calls temporarily blocks process
           Product: uClibc
           Version: 0.9.33.2
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Other
          Assignee: unassigned at uclibc.org
          Reporter: mjl-s at t-online.de
                CC: uclibc-cvs at uclibc.org
  Target Milestone: ---

On an AMD-Elan-based system with Linux 2.6.32 and uClibc 0.9.33.2, a flood of
syslog() calls in a real-time process led to the process freezing up for about
two seconds.

This was caused by the following code in libc/misc/syslog/sylog.c, vsyslog():

 write_err:
        /*
         * Output the message to the console; don't worry about blocking,
         * if console blocks everything will.  Make sure the error reported
         * is the one from the syslogd failure.
         */
        /* should mode be O_WRONLY | O_NOCTTY? -- Uli */
        /* yes, but in Linux "/dev/console" never becomes ctty anyway -- vda */
        if ((LogStat & LOG_CONS) &&
            (fd = open(_PATH_CONSOLE, O_WRONLY | O_NOCTTY)) >= 0) {
                p = strchr(tbuf, '>') + 1;
                last_chr[0] = '\r';
                last_chr[1] = '\n';
                (void)write(fd, p, last_chr - p + 2);
                (void)close(fd);
        }

The system in question had a serial console, configured for 19.2 kb/s with 8N1.
The kernel's internal buffer for that console has a size of UART_XMIT_SIZE,
i.e., 4096 bytes. Transmitting 4096 bytes at 19.2 kb/s with 8N1 requires 2.1
seconds.

Adding O_NONBLOCK to the open() call above fixed the problem, i.e., the comment
a few lines above that call seems unnecessarily optimistic.

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