svn commit: r211176 - in head/sys: amd64/amd64 i386/i386

[Kostik Belousov]

On Wed, Aug 11, 2010 at 04:29:21PM +0200, Attilio Rao wrote:
> 2010/8/11 Kostik Belousov <kostikbel at gmail.com>:
> > On Wed, Aug 11, 2010 at 01:21:46PM +0200, Attilio Rao wrote:
> >> 2010/8/11 Kostik Belousov <kostikbel at gmail.com>:
> >> > On Wed, Aug 11, 2010 at 10:51:27AM +0000, Attilio Rao wrote:
> >> >> Author: attilio
> >> >> Date: Wed Aug 11 10:51:27 2010
> >> >> New Revision: 211176
> >> >> URL: http://svn.freebsd.org/changeset/base/211176
> >> >>
> >> >> Log:
> >> >>   IPI handlers may run generally with interrupts disabled because they
> >> >>   are served via an interrupt gate.
> >> >>
> >> >>   However, that doesn't explicitly prevent preemption and thread
> >> >>   migration thus scheduler pinning may be necessary in some handlers.
> >> >>   Fix that.
> >> >
> >> > How the preemption is supposed to happen ? Aside from the explicit
> >> > calls to mi_switch() from e.g. critical_exit().
> >>
> >> IIRC it should be hardclock() willing to schedule the softclock(). It
> >> is the critical_exit() in the thread_unlock() that may trigger it
> >> (sorry for not digging more, it was a while back that I hacked this
> >> part, but I guess you can verify on your own).
> >> We already have other points within the kernel that take care of
> >> dealing with preemption/migration like lapic_handle_timer(), for
> >> example.
> >
> > Right, and if the interrupts are indeed disabled, I do not see how
> > the preemption may be triggered in the fragments like
> >        cpu = PCPU_GET(cpuid);
> >        cpumask = PCPU_GET(cpumask);
> 
> I don't recall all the details and I have no time to dig now. However,
> also spinlock_enter() does disable explicitly preemption via
> critical_enter() after have disabled the interrupts.
> Let me CC jhb as he implemented spinlock_enter() and may have a clue
> about how preemption can happen with interrupts disabled.

spinlock_enter() disables preemption to handle the recursive
calls to spinlock_enter/leave, I think, to prevent switch on
leave.

I do understand the reason for your change.
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