Subject : sshd hangs on close
Submitter : Matthias Urlichs <email@example.com>
Date : 2008-08-30 9:18
References : http://marc.info/?l=linux-kernel&m=122008800512864&w=4
This entry is being used for tracking a regression from 2.6.26. Please don't
close it until the problem is fixed in the mainline.
Unfortunately, the problem seems to have permanently vanished after the affected machine has been rebooted with a known-good kernel, and has since resisted my attempts at reproducing it.
I am able to reproduce this problem on one machine with 2.6.32 (rhel6). It appears to be due to needing a kernel-thread 'events' to get cpu time on a foreign cpu. E.g. if sshd is running on cpu0, and cpu1 is shielded with a FIFO-99 process starving the kernel-thread 'events/1' (non-RT), sshd on cpu0 hangs with wchan=flush_cpu with the last strace being closing the filedescriptor of /dev/ptmx.
Question1 - why does cpu0 wait for events/1?
Question2 - why does sshd's close /dev/ptmx trigger this?
Question3 - what exactly is wchan=cpu_flush indicating?