/proc/kmsg giving eof on blocking read

[Jan Schmidt <list.lkml@jan-o-sch.net>]

Hi,

I'm currently debugging something in btrfs in good old printk style, generating
around 10MB/min. I'm seeing /proc/kmsg returning eof on a blocking read (and,
side note, syslog-ng won't reopen it, effectively stopping logging kernel
messages silently).

I'm using kernel 3.6.0+ from cmason's tree, which is Linux 3.6.0 (commit
a0d271cbfed1dd50278c6b06bead3d00ba0a88f9) plus the Btrfs code for 3.7 (commit
c37b2b6269ee4637fb7cdb5da0d1e47215d57ce2).

I suspect it has something to do with the data I'm passing to printk. It happens
anywhere from several times per second to once every twenty minutes.

As a workaround (and proof), I'm currently using:

# perl -we 'use Fcntl; sysopen(K, "/proc/kmsg", O_RDONLY) or die "open $!";
while (1) {while ($ret = sysread(K, $buf, 8192)) {print $buf} print STDERR
scalar(localtime(time)), " oops $ret\\n";}' >> /var/tmp/kern

-Jan