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

[Kostik Belousov]

On Wed, Aug 11, 2010 at 05:46:46PM +0300, Kostik Belousov wrote:
> 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.
do _not_ understand. sorry.


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