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

[Attilio Rao]

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()?

Attilio


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