svn commit: r234952 - in head/sys: kern sys

Konstantin Belousov kostikbel at gmail.com
Thu May 3 11:49:22 UTC 2012


On Thu, May 03, 2012 at 12:02:08PM +0100, Attilio Rao wrote:
> 2012/5/3, Konstantin Belousov <kib at freebsd.org>:
> > Author: kib
> > Date: Thu May  3 10:38:02 2012
> > New Revision: 234952
> > URL: http://svn.freebsd.org/changeset/base/234952
> >
> > Log:
> >   When callout_reset_on() cannot immediately migrate a callout since it
> >   is running on other cpu, the CALLOUT_PENDING flag is temporarily
> >   cleared. Then, callout_stop() on this, in fact active, callout fails
> >   because CALLOUT_PENDING is not set, and callout_stop() returns 0.
> >
> >   Now, in sleepq_check_timeout(), the failed callout_stop() causes the
> >   sleepq code to execute mi_switch() without even setting the wmesg,
> >   since the switch-out is supposed to be transient. In fact, the thread
> >   is put off the CPU for full timeout interval, instead of being put on
> >   runq immediately.  Until timeout fires, the process is unkillable for
> >   obvious reasons.
> >
> >   Fix this by marking the migrating callouts with CALLOUT_DFRMIGRATION
> >   flag. The flag is cleared by callout_stop_safe() when the function
> >   detects a migration, besides returning the success. The softclock()
> >   rechecks the flag for migrating callout and cancels its execution if
> >   the flag was cleared meantime.
> 
> Can you please clarify why you cannot simply drop the deferred
> migration in the case !CALLOUT_PENDING in callout_stop_safe()?

I probably can, I think I went with the route of committed patch
because it is slightly less work. Also, the comment in the while()
loop suggested me to rely on softclock.
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