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

[Konstantin Belousov]

On Thu, May 03, 2012 at 02:14:20PM +0100, Attilio Rao wrote:
> 2012/5/3, Konstantin Belousov <kostikbel at gmail.com>:
> > 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.
> 
> I don't think this is more work at all, the attached patch
> (pre-r234952, untested) should address it properly in few than 10
> lines:
> http://www.freebsd.org/~attilio/callout_cancel_mig_stop.patch
> 
> without the need to add further flags and re-using existing mechanisms.

(cc->cc_curr != c) is not the case which caused the issue. It might be
needed to treatened this way, but the reported case is opposite.
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