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 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.

Yes, of course, because the migration handover happens in the same
critical context of cc->cc_curr == c, but now I wonder if this fix is
really right.

It seems to me that in the case you describe callout_stop() must
return 0 and the migration must not be cancelled because the callout
is not stopped. It is not stopped not because of the deferred
migration but because cc->cc_curr == c. It seems a perfectly valid
situation to me.

Probabilly the bug is in the sleepq use of this mechanism?

Attilio


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