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

[Attilio Rao]

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.

Attilio


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