unbounded sleep on [fifoow] while open a named pipe: is it a feature?

Sergey Kandaurov pluknet at gmail.com
Fri Jan 28 09:43:40 UTC 2011


That's FreeBSD 8.1-RELEASE i386 (w/o debug).

It's observed for bash processes which end up in unbounded sleep
in [fifoow] wchan while executing the next script:




exec  > >(logger -p "$LOG_FACILITY" -t "$LOG_TOPIC_OUT" )
exec 2> >(logger -p "$LOG_FACILITY" -t "$LOG_TOPIC_ERR" )
eval exec $1

It's used to call $1 and redirect its stdout and stderr streams
to different files. Bash implements this functionality by creating
on every execution a new named pipe stored at /var/tmp/ directory.
This script is used to run periodically from cron(8). Note, that bash
has a misfeature to not delete those named pipes. With a high creation
frequency of named pipes (several per minute) it eventually ends
up with a large number [~137000] of existing named pipes, when it
was found that a number [~44] of bash processes sleep on [fifoow],
which is guessed to be likely caused by the number of fifo pipes created.

For example, this one sleeps for about 1.5 days:
 1001 78809 78803   0  50  0  4564  1952 fifoow Is    ??    0:00.00
/usr/local/bin/bash ./logger_wrapper.sh ./sync_overlords.pl

# procstat -f 78809
78809 bash                1 f - -w------   1       0 -   /var/tmp/sh-np-[random]

# procstat -kk 78809
  PID    TID COMM             TDNAME           KSTACK
78809 100733 bash             -                mi_switch+0x12a
sleepq_switch+0xcc sleepq_catch_signals+0x5b sleepq_wait_sig+0x18
_sleep+0x28c fifo_open+0x718 VOP_OPEN_APV+0x42 vn_open_cred+0x52b
vn_open+0x3b kern_openat+0x125 kern_open+0x35 open+0x30 syscall+0x2d3


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